only show go_version segment if inside GOPATH
only show go_version segment if inside GOPATH

As of Go v1.8 GOPATH need not be set, as it will default to $HOME/go. By using `go env GOPATH` to retrieve the value first, this technique will work regardless of whether GOPATH is currently set by the user.
rjorgenson/powerlevel9k,dritter/powerlevel9k,rjorgenson/powerlevel9k,natemccurdy/powerlevel9k,bhilburn/powerlevel9k,natemccurdy/powerlevel9k,bhilburn/powerlevel9k,dritter/powerlevel9k