Swift 5.3 support

Add Swift 5.3 support (#51)

* Add instructions for Swift 5.3

* Make 5.3 the default Swift version

Swift 5.2.5 support

Update versions, clarify test discovery in readme (#50)

* Update versions, clarify test discovery in readme

* Set 5.2.5 as the default Swift version

* Add local 5.2.5 support

Swift 5.2.4 support

• The buildpack now supports and defaults to Swift 5.2.4.
• Heroku CI is now supported: the buildpack runs swift test on the application project when set up, optionally extendable with pre-test and post-test hooks (credits to @markshiz).

Swift 5.2.3 support

The buildpack now supports and defaults to Swift 5.2.3.

Swift 5.2.2 support

The buildpack now supports and defaults to Swift 5.2.2.

Bonus: trying to use Swift 5.2 with the cedar-14 stack (Ubuntu 14.04) will result in a clear error message with the options of resolution.

Swift 5.2.1 as default and new features

• The default Swift version will be 5.2.1, with the promise that it will be updated quickly as new toolchains drop.

  Living legacy projects can still pin old versions manually.

• On top of the .swift-version file, the SWIFT_VERSION configuration variable can be used to specify the pinned Swift version.

  This helps consistency with other build parameters and may be easier to explain than the hidden file.

• Clang is no longer installed for Swift 5.x.

  It was just wasted build time and cache space anyway.

• The README has a practical example for SWIFT_BUILD_FLAGS.

Swift 5.2 support

5.2-release

Add URLs for Swift 5.2 release

Fix URLs of the 2020-03-23a snapshot

5.2-mar23-fix

Fix URLs of the 03-23 snapshot...

Swift 5.2-DEVELOPMENT-SNAPSHOT-2020-03-23-a support

5.2-mar23

Add URLs for the March 23 snapshot

Swift 5.2-DEVELOPMENT-SNAPSHOT-2020-03-20-a support

5.2-mar20

Add URLs for the latest 5.2 development snapshot