Update cocoapod install to be compatible with cocoapod 1.x as it now requires a concrete target.

Search problem takes about five minutes, but it is much easier to see commands that just doing the right thing.

Xcode 7.1 is out in the App Store, the comment related to rdar://22307360 is no longer needed

I felt the description and instructions for *Carthage* weren't accurate.

- reworded the introduction to *Carthage* to be closer to how it describes itself.
- added project integration step instruction for *Carthage* based on its documentation.

Now that the `swift-2.0` branch has been merged and deleted the migration guide is located on the `master` branch.