[pod init] Set `source` directive to use `master` repo URL.

Related to:
* https://github.com/CocoaPods/Core/issues/170
* https://github.com/CocoaPods/CocoaPods/pull/2513
* https://github.com/CocoaPods/CocoaPods/issues/2515#issuecomment-56801012

/cc @segiddins @fabiopelosin
https://github.com/CocoaPods/CocoaPods/issues/2519