Fix Travis CI tests.

- Fix Travis CI (step one) by removing `brew install xctool` command from `travis.yml`. Apparently Travis machines already have this tool installed, and attempting to install it again causes a failure. Builds [112 through 117](https://travis-ci.org/allending/Kiwi/pull_requests) failed for this reason.
- Fix Travis CI (step two) by passing `-sdk iphonesimulator` argument to `xctool`. This is in response to an error emitted by `xctool` when the argument isn't passed: "failed: Testing with the 'iphoneos' SDK is not yet supported.  Instead, test with the simulator SDK by setting '-sdk iphonesimulator'."
- Wrap calls to `libunwind.h` methods in `__arm__` macro. I know, I know--a lot of my PRs against this repo include some shady macro usage. But these methods have been causing failures for quite a while now; see issue #335.