Podfile.lock should be kept in source control

[aaronschachter]

• [x] Remove Podfile.lock from .gitignore and add back into project after a pod install.

From https://guides.cocoapods.org/using/using-cocoapods.html#benefits-of-ignoring-the-pods-directory:

Whether or not you check in the Pods directory, the Podfile and Podfile.lock should always be kept under version control.

[aaronschachter]

Closed by #1025