[Podspec] Specify extensions of source files.
[Podspec] Specify extensions of source files.

jkrzemie/RSKImageCropper,ruslanskorb/RSKImageCropper,tonibagur/RSKImageCropper,shenhzou654321/RSKImageCropper,ruslanskorb/RSKImageCropper,cristiangu/RSKImageCropper,cfiken/RSKImageCropper,andrew804/RSKImageCropper