Update version to 2.2.2 for XCTest support.

Changelog:

- Add class name to failure message in equality checks. (5061ae2ead443a783c99c323bb2a4991b8414053)
- Fix ARM compilation errors. (d47434b13e8b32a82de35884b1dee0a09c1f046f)
- Silence `atos` deprecation warnings output when example groups were built. (f5aec48ac16e370082e0e18a842a07e9118bba1f)
- Add "Kiwi/XCTest" subspec for XCTest unit test bundle support. (672bd210f0227ba92fb7b895dcdf6600ec126089)

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A core member is going to need to tag `head` as `2.2.2`. I can update the CocoaPods spec once that's done, or the same person could do it/submit a pull request to https://github.com/CocoaPods/Specs.