Bump Podspec to 2.2.4

Hi, I was hoping to release a more recent Podspec for Kiwi. Based on the commit history, [its been 5 months since a release](https://github.com/allending/Kiwi/commits/master/Kiwi.podspec). Since then a major [issue](https://github.com/allending/Kiwi/issues/390) for me has been fixed, so I was hoping to take advantage of that. 

I wasn't sure if there needs to be extensive testing or something done before making a release; if so you can just close this PR.