Default to go1.7
Default to go1.7

Drop support for go1.5

michael-go/heroku-buildpack-go,michael-go/heroku-buildpack-go,titanous/heroku-buildpack-go,heroku/heroku-buildpack-go,cloudfoundry/go-buildpack,cloudfoundry/go-buildpack,cloudfoundry/go-buildpack,heroku/heroku-buildpack-go