Carthage dependency redirection

[SimonRice]

Pop has been moved to facebookarchive/pop.

While Carthage does redirect requests for downloads, the underlying URLSession authentication for Github's API doesn't always follow the redirect. This can cause problems on some CI systems, notably bitrise. In this case, the lack of redirect means Carthage has to rebuild Pop when a perfectly good binary is available.

This PR sets pop's source in both the Cartfile & its resolved file accordingly so such a redirect isn't needed.