travis: upgrade Go version (#855)
travis: upgrade Go version (#855)

1.9 was released on 24 August 2017:
https://golang.org/doc/go1.9

1.8.3 was released on 24 May 2017:
https://golang.org/doc/devel/release.html#go1.8.minor

This keeps up with the policy of only testing the
current & previous release.
robphoenix/exercism-go,exercism/xgo,leenipper/xgo,exercism/xgo,robphoenix/exercism-go,leenipper/xgo