Better podspec source examples

After discussion in CocoaPods/CocoaPods#2334

Before merging, I’d like to hear your opinion about [Using the **version of the Pod** to identify the Git commit](https://github.com/CocoaPods/Core/blob/3b72760a736bd9733a42912882009d9e85838a6d/lib/cocoapods-core/specification/dsl.rb#L249-L252):
> :commit => "v#{spec.version}"

Does this actually work? This seems very weird to me.