Cut down on OS X builds on Travis

[Arcanemagus]

The default CI configuration for Travis-CI runs 2 parallel builds on OS X, this can cause exceptionally long queue times as it seems that Travis-CI currently only allocates 100 containers for all Open Source OSX builds:

[UziTech]

we could use allow failures. That way the tests would still be run but not slow down PR test validation

[Arcanemagus]

allow_failures just means that that specific job doesn't fail the entire build, it has nothing to do with the queue times which are 15 min on a good day. I've left it enabled for the few providers that have shown to be problematic there (linter-ruby for example), but for the most part these builds just slow everything down for no good reason.

[UziTech]

With allow_failures we could use fast_finish to tell travis not to wait for the allowed failures.