Change podspec outermost source_files to only include TransformerKit/TransformerKit.h

Previously `s.source_files = 'TransformerKit'` included all files under TransformerKit, and then each subspec also added its own files. The Pods-TransformerKit Xcode target double-included each .m file from the subspecs when using plain `pod 'TransformerKit'`, causing linker errors. Change to just include TransformerKit.h, and then each subspec is responsible for its own files.

The spec also needs to be fixed in the CocoaPods/Specs repo, but seemed I should report here first.