Verify dependencies are met when validating a submitted pod

[kylef]

People shouldn't be able to submit a pod we don't know about. Such as a pod that depends on a private pod. It will pass linting locally because I might have the private repo.

[fabiopelosin]

:+1:

[kylef]

Related: https://github.com/CocoaPods/cocoapods-trunk/issues/28