Add PromiseKit dependency

- Added PromiseKit dependency

Carthage/Checkouts/PromiseKit/Tests/A+/README.md

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+Resources
+=========
+* https://github.com/promises-aplus/promises-tests
+
+
+Skipped
+=======
+* 2.3.3: Otherwise, if x is an object or function.
+  This spec is a NOOP for Swift:
+  - We have decided not to interact with other Promises A+ implementations
+  - functions cannot have properties
+* 2.3.3.4: If then is not a function, fulfill promise with x.
+  - See: The 2.3.4 suite.