Mantle's transformerWithBlock is Deprecated

`[MTLValueTransformer transformerWithBlock:]` [is deprecated](https://github.com/Mantle/Mantle/blob/3098f8e7bb24ca3132420a34ad38c1acec76d743/Mantle/MTLValueTransformer.h#L44). Now we are supposed to use `transformerUsingForwardBlock` or `transformerUsingReversibleBlock`. There are about 32 instances of `transformerWithBlock` in RedditKit which are now showing as warnings.