Changes:

- Backwards incompatible changes for Swift 2.0.
- XCTestObservationCenter is now a public XCTest API, although it
  doesn't expose "suspend observation". Quick tests now use the public
  APIs where applicable.
- Added Nimble to Quick Focused OSX Test Target in order to fix a CI
  failure.

Removed:

- Removed Failure.swift, which was no longer being used within the framework
  (probably) as of 6a68691, and was not a class that Quick consumers should
  have been relying upon.