Default to `SOURCE_VERSION` environment variable
Default to `SOURCE_VERSION` environment variable

Sing-Li/go-buildpack,virtmerlin/go-buildpack,Sing-Li/go-buildpack,titanous/heroku-buildpack-go,cloudfoundry/go-buildpack,stevenwilkin/go-buildpack,Jonty/go-buildpack,michael-go/heroku-buildpack-go,hpcloud/go-buildpack,venicegeo/go-pdal-buildpack,half-ogre/go-buildpack,enkoder/heroku-buildpack-go,Jonty/go-buildpack,Sing-Li/go-buildpack,stevenwilkin/go-buildpack,cloudfoundry/go-buildpack,loafoe/go-buildpack,Sing-Li/go-buildpack,heroku/heroku-buildpack-go,cloudfoundry/go-buildpack,loafoe/go-buildpack,heroku/heroku-buildpack-go,loafoe/go-buildpack,Sing-Li/go-buildpack,Jonty/go-buildpack,hpcloud/go-buildpack,stevenwilkin/go-buildpack,Sing-Li/go-buildpack,michael-go/heroku-buildpack-go,hpcloud/go-buildpack,half-ogre/go-buildpack,venicegeo/go-pdal-buildpack,venicegeo/go-pdal-buildpack,half-ogre/go-buildpack,virtmerlin/go-buildpack,Sing-Li/go-buildpack,virtmerlin/go-buildpack,Sing-Li/go-buildpack