Use HEROKU_SLUG_COMMIT instead of SOURCE_VERSION
Use HEROKU_SLUG_COMMIT instead of SOURCE_VERSION

HEROKU_SLUG_COMMIT is available without the use of a third-party
buildpack

JuliaCI/travis-build,andyli/travis-build,JuliaCI/travis-build,craigcitro/travis-build,craigcitro/travis-build,kevmoo/travis-build,andyli/travis-build,kevmoo/travis-build