Fix bug from Pull request #395

    #395 broke oh-my-zsh for users who disable check-for-updates

oh-my-zsh.sh

 # Check for updates on initial load...
-if [ "$DISABLE_AUTO_UPDATE" = "true" ]
+if [ "$DISABLE_AUTO_UPDATE" != "true" ]
 then
-  return
-else
   /usr/bin/env zsh $ZSH/tools/check_for_upgrade.sh
 fi