Fix explicit import of Mantle’s value transformer.

The explicit import of `MTLValueTransformer.h` introduced in #71 for compatibility with CocoaPods led to a compile time error when building the project from the Xcode workspace as the header can’t be found. This patch aims to fix that but as I do not personally use CocoaPods or are very familiar with it I do not know if this would break what was fixed in #71.

As such I would perhaps be nice to hear from for example @gfontenot (or anyone else familiar with CocoaPods) whether this would cause any issues for CocoaPods users.