`rake pull` should warn if Rainforest itself was updated

[See here for discussion](https://github.com/CocoaPods/Rainforest/commit/f677f3fa887d93205de980e3f296dcf1c135ef0d#commitcomment-6368700) and [here for implementation example](https://github.com/CocoaPods/Rainforest/blob/master/Rakefile#L132)