Cleanup Rakefile, run OSX tests only on >= Mavericks

The PR affects

__.travis.yml__:
- move test decisions to `Rakefile`
- remove `brew update` since it was only used for `xctool`

__Rakefile__:
- use `FileUtils` instead of `system(%Q(strings strings))`
- separate private helpers from tasks
- check OSX version and run tests if it's >= 10.9
- exit with the same status code as `xcodebuild`