Change default go version from 1.7.5 to 1.8.1 [#143721781]
Change default go version from 1.7.5 to 1.8.1 [#143721781]

Signed-off-by: Nikita Rathi <c9767bf2fec5c766a4bbec0247a4fe5653c411cf@pivotal.io>

cloudfoundry/go-buildpack,cloudfoundry/go-buildpack,cloudfoundry/go-buildpack