Make rvm prompt function a bit cleaner

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)"
+  [[ ! -z $ruby_version ]] && echo "($ruby_version)"
 }
-
-