Run unit tests on multiple SDKs.

I think it might be nice to ensure that Kiwi works for iOS SDK 6.1 (a friend of mine uses an old MacBook that can't run Mavericks, and so can't use iOS SDK 7.0). Feel free to close this PR if that seems like more trouble than it's worth, though.

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By the way, I encountered an interesting false positive with `xcpretty` when attempting to run against `iphonesimulator5.1`: `xcpretty` exited with a `0` status code despite the fact that the `xcodebuild` command failed. You can [see the full output here](https://travis-ci.org/modocache/Kiwi/jobs/20285282). I'm not sure, but it might be related to https://github.com/supermarin/xcpretty/pull/66.