Don't ignore the Pods directory

[zwaldowski]

Due to the rapidly changing nature of the Swift language itself, the CocoaPods ecosystem, and all the involved dependencies, it becomes more important that a given commit be a snapshot of a functioning source tree at a given time.

Example: There are master and swift-next branches. I want to add a feature to both of them. Switching branches requires running pod install each time, which may be problematic if the dependencies (or nested, transitive, etc. dependencies) change across the branches.