[v2] [iOS] Remove OCMock files from being compiled into pod (#4272)

* Remove OCMock files from being compiled into pod

* Add prepare script to build source when repo is cloned

* Revert package.json change

ReactNativeNavigation.podspec

 
   s.source       = { :git => "https://github.com/wix/react-native-navigation.git", :tag => "#{s.version}" }
   s.source_files  = "lib/ios/**/*.{h,m}"
-  s.exclude_files  = "lib/ios/ReactNativeNavigationTests/**/*.*"
+  s.exclude_files  = "lib/ios/ReactNativeNavigationTests/**/*.*", "lib/ios/OCMock/**/*.*"
 
   s.dependency 'React'
   s.frameworks = 'UIKit'