Merge pull request #1330 from trobrock/hide-rvm-prompt

Don't show empty parens if you are using your system ruby

lib/rvm.zsh

 # get the name of the branch we are on
 function rvm_prompt_info() {
   ruby_version=$(~/.rvm/bin/rvm-prompt 2> /dev/null) || return
-  echo "($ruby_version)"
+  [[ ! -z $ruby_version ]] && echo "($ruby_version)"
 }
-
-