Add Heroku CI variable for git commit SHA

Needs a similar approach to other CI services, as Heroku CI doesn't initialise a git repository on it's CI platform.

https://devcenter.heroku.com/articles/heroku-ci#environment-variables-env-key

Author: andycroll

lib/codacy/git.rb

@@ -8,6 +8,7 @@ def self.commit_id
           ENV['CIRCLE_SHA1'] ||
           ENV['CI_COMMIT_ID'] ||
           ENV['WERCKER_GIT_COMMIT'] ||
+          ENV['HEROKU_TEST_RUN_COMMIT_VERSION'] ||
           git_commit
 
       commit