OSX: Fail Travis run on Analyze warnings

Scripting required because xctool fails to report analyze failures
properly:
https://github.com/facebook/xctool/issues/436
OSX: Don't do 'brew upgrade' as part of Travis run

Its purpose was to get the newest version of xctool installed.

We're fine with xctool 0.2.0 so upgrading isn't necessary.

We had downloads time out on us and breaking the build for non-code
reasons as well, so removing the Homebrew interaction seems like the
obvious thing to do.
Merge pull request #2 from walles/fail-travis-on-analyze-warnings

OSX: Fail Travis run on Analyze warnings