Merge pull request #273 from blueyed/hub-alias-only-with-ruby

Minor fix for the github plugin: check for `ruby`.

plugins/github/github.plugin.zsh

-# hub alias from defunkt
-# https://github.com/defunkt/hub
-if [ "$commands[(I)hub]" ]; then
+# Setup hub function for git, if it is available; http://github.com/defunkt/hub
+if [ "$commands[(I)hub]" ] && [ "$commands[(I)ruby]" ]; then
     # eval `hub alias -s zsh`
     function git(){hub "$@"}
 fi