Specify KIF subspecs to avoid iOS 6 problems.

KIF defaults to its `XCTest` subspec, which means that for iOS 6 apps,
the `SenTestingKit` subspec of Kiwi-KIF will fail to launch, as it can’t
find the XCTest framework.
Merge pull request #17 from SlaunchaMan/patch-1

Specify KIF subspecs to avoid iOS 6 problems.