Set up codecov (#9)

- Add a token into Travis CI ENV vars
- Add a badge to README.md

.travis.yml

@@ -1,3 +1,6 @@
+env:
+  global:
+  - secure: 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
 language: ruby
 rvm:
 - 2.3

README.md

@@ -1,6 +1,7 @@
 # sorbet-coerce
 [![Gem Version](https://badge.fury.io/rb/sorbet-coerce.svg)](https://badge.fury.io/rb/sorbet-coerce)
 [![Build Status](https://travis-ci.com/chanzuckerberg/sorbet-coerce.svg?branch=master)](https://travis-ci.com/chanzuckerberg/sorbet-coerce)
+[![codecov](https://codecov.io/gh/chanzuckerberg/sorbet-coerce/branch/master/graph/badge.svg)](https://codecov.io/gh/chanzuckerberg/sorbet-coerce)
 
 A type coercion lib works with [Sorbet](https://sorbet.org)'s static type checker and type definitions; raises an error if the coercion fails.