ci: R.I.P. Travis CI - Long live GitHub Actions

Follows:
- #544

Outstanding tasks:
- #549
- #548

(cherry picked from develop commit 434e78f)

CMakeLists.txt

@@ -41,7 +41,7 @@ message(STATUS "Boost.GIL: Require C++${CMAKE_CXX_STANDARD}")
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 set(CMAKE_CXX_EXTENSIONS OFF)
 
-# Avoid warnings flood on Travis CI, AppVeyor, CircleCI, Azure Pipelines
+# Avoid warnings flood from CI builds
 if(DEFINED ENV{CI} OR DEFINED ENV{AGENT_JOBSTATUS} OR DEFINED ENV{GITHUB_ACTIONS})
   set(BOOST_GIL_BUILD_CI ON)
   message(STATUS "Boost.GIL: Turning off detailed compiler warnings for CI build short log")

CONTRIBUTING.md

@@ -233,10 +233,10 @@ git push <username> feature/foo
 Finally, sign in to your GitHub account and
 [create a pull request](https://help.github.com/articles/creating-a-pull-request/).
 
-Your pull request will be automatically built and tests will run on Travis CI
-and AppVeyor (see [README](README.md) for builds status). Please, keep an eye
-on those CI builds and correct any problems detected in your contribution
-by updating your pull request.
+Your pull request will be automatically built and tests will run on GitHub ACtions,
+Azure Pipelines, AppVeyor and Circle CI (see [README](README.md) for builds status).
+Please, keep an eye on those CI builds and correct any problems detected in your
+contribution by updating your pull request.
 
 ### 5. Update your pull request
 
@@ -300,7 +300,7 @@ request from reviewer, just add new commits:
 cd libs/gil
 git checkout feature/foo
 git add -A
-git commit -m "Fix build Travis CI failures"
+git commit -m "Fix CI build failure"
 git push <username> feature/foo
 ```
 

README.md

@@ -10,10 +10,10 @@
 [![Conan](https://img.shields.io/badge/on-conan-blue.svg)](https://bintray.com/bincrafters/public-conan/boost_gil%3Abincrafters)
 [![Vcpkg](https://img.shields.io/badge/on-vcpkg-blue.svg)](https://github.com/Microsoft/vcpkg/tree/master/ports/boost-gil)
 
-Documentation | AppVeyor        | Azure Pipelines | Travis CI       | CircleCI        | Regression
---------------|-----------------|-----------------|-----------------|-----------------|------------
-[![develop](https://img.shields.io/badge/doc-develop-blue.svg)](https://boostorg.github.io/gil/develop/) | [![AppVeyor](https://ci.appveyor.com/api/projects/status/w4k19d8io2af168h/branch/develop?svg=true)](https://ci.appveyor.com/project/stefanseefeld/gil/branch/develop) | [![Azure](https://dev.azure.com/boostorg/gil/_apis/build/status/boostorg.gil?branchName=develop)](https://dev.azure.com/boostorg/gil/_build/latest?definitionId=7&branchName=develop) | [![Travis](https://travis-ci.org/boostorg/gil.svg?branch=develop)](https://travis-ci.org/boostorg/gil) | [![CircleCI](https://circleci.com/gh/boostorg/gil/tree/develop.svg?style=shield)](https://circleci.com/gh/boostorg/workflows/gil/tree/develop) | [![gil](https://img.shields.io/badge/gil-develop-blue.svg)](http://www.boost.org/development/tests/develop/developer/gil.html)
-[![master](https://img.shields.io/badge/doc-master-blue.svg)](https://boostorg.github.io/gil/) | [![AppVeyor](https://ci.appveyor.com/api/projects/status/w4k19d8io2af168h?svg=true)](https://ci.appveyor.com/project/stefanseefeld/gil/branch/master) | [![Azure](https://dev.azure.com/boostorg/gil/_apis/build/status/boostorg.gil?branchName=master)](https://dev.azure.com/boostorg/gil/_build/latest?definitionId=7&branchName=master) | [![Travis](https://travis-ci.org/boostorg/gil.svg?branch=master)](https://travis-ci.org/boostorg/gil) | [![CircleCI](https://circleci.com/gh/boostorg/gil/tree/master.svg?style=shield)](https://circleci.com/gh/boostorg/workflows/gil/tree/master) | [![gil](https://img.shields.io/badge/gil-master-blue.svg)](http://www.boost.org/development/tests/master/developer/gil.html)
+Documentation | GitHub Actions | Azure Pipelines | CircleCI        | Regression
+--------------|----------------|-----------------|-----------------|------------
+[![develop](https://img.shields.io/badge/doc-develop-blue.svg)](https://boostorg.github.io/gil/develop/) | [![GitHub Actions](https://github.com/boostorg/gil/workflows/CI/badge.svg?branch=develop)](https://github.com/boostorg/gil/actions?query=branch:develop) | [![AppVeyor](https://ci.appveyor.com/api/projects/status/w4k19d8io2af168h/branch/develop?svg=true)](https://ci.appveyor.com/project/stefanseefeld/gil/branch/develop) | [![Azure](https://dev.azure.com/boostorg/gil/_apis/build/status/boostorg.gil?branchName=develop)](https://dev.azure.com/boostorg/gil/_build/latest?definitionId=7&branchName=develop) | [![CircleCI](https://circleci.com/gh/boostorg/gil/tree/develop.svg?style=shield)](https://circleci.com/gh/boostorg/workflows/gil/tree/develop) | [![gil](https://img.shields.io/badge/gil-develop-blue.svg)](http://www.boost.org/development/tests/develop/developer/gil.html)
+[![master](https://img.shields.io/badge/doc-master-blue.svg)](https://boostorg.github.io/gil/) | [![GitHub Actions](https://github.com/boostorg/gil/workflows/CI/badge.svg?branch=master)](https://github.com/boostorg/gil/actions?query=branch:master) | [![AppVeyor](https://ci.appveyor.com/api/projects/status/w4k19d8io2af168h?svg=true)](https://ci.appveyor.com/project/stefanseefeld/gil/branch/master) | [![Azure](https://dev.azure.com/boostorg/gil/_apis/build/status/boostorg.gil?branchName=master)](https://dev.azure.com/boostorg/gil/_build/latest?definitionId=7&branchName=master) | [![CircleCI](https://circleci.com/gh/boostorg/gil/tree/master.svg?style=shield)](https://circleci.com/gh/boostorg/workflows/gil/tree/master) | [![gil](https://img.shields.io/badge/gil-master-blue.svg)](http://www.boost.org/development/tests/master/developer/gil.html)
 
 # Boost.GIL
 

test/Jamfile

@@ -16,7 +16,7 @@ import regex ;
 import sequence ;
 import testing ;
 
-# Avoid warnings flood on Travis CI, AppVeyor, CircleCI, Azure Pipelines, GitHub Actions
+# Avoid warnings flood from CI builds
 if ! [ os.environ CI ] && ! [ os.environ AGENT_JOBSTATUS ] && ! [ os.environ GITHUB_ACTIONS ]
 {
   DEVELOPMENT_EXTRA_WARNINGS =