Fixing the rvm_prompt_info command, now it will not show empty parens if no...

Fixing the rvm_prompt_info command, now it will not show empty parens if no rvm is currently being used

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
+  if [ -z $ruby_version ]
+  then
+    return
+  fi
   echo "($ruby_version)"
 }