When linting, only allow dependencies from master

[kylef]

Suppress the following warning and force master to be used.

[!] The use of implicit sources has been deprecated. To continue using all of the sources currently on your machine, add the following to the top of your Podfile:

    source 'https://github.com/CocoaPods/Specs.git'

[segiddins]

Validator needs a way to take only a given array of sources, then a change at https://github.com/CocoaPods/cocoapods-trunk/blob/master/lib/pod/command/trunk.rb#L336

[kylef]

Yes, those changes will probably be needed for CocoaPods/CocoaPods#2543 too.