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.

powerlevel9k.zsh-theme

@@ -744,9 +744,11 @@ prompt_docker_machine() {
 # GO prompt
 prompt_go_version() {
   local go_version
+  local go_path
   go_version=$(go version 2>/dev/null | sed -E "s/.*(go[0-9.]*).*/\1/")
+  go_path=$(go env GOPATH)
 
-  if [[ -n "$go_version" ]]; then
+  if [[ -n "$go_version" && "${PWD##$go_path}" != "$PWD" ]]; then
     "$1_prompt_segment" "$0" "$2" "green" "255" "$go_version"
   fi
 }