Exclude the podspec from pod

The `JFBCrypt.*` in `source_files` is picking up `JFBCrypt.podspec` which, since it's interpreted as a source file, ends up in the Pods project's Compile Sources list. As a podspec isn't a source file Xcode pops a warning stating as such.

This change excludes `JFBCrypt.podspec` from the `source_files`. I've also [filed an issue](https://github.com/CocoaPods/CocoaPods/issues/2285) with CocoaPods to automatically prevent podspecs from being included in source files.