README: update references to go 1.8 instead of 1.7
README: update references to go 1.8 instead of 1.7

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