Temporarily allows osx homebrew Python 3.6 to fail (#572)

* Temporarily allows osx homebrew Python 3.6 to fail.

https://github.com/pybind/pybind11/pull/570#issuecomment-269120613

"Homebrew just got Python 3.6 (brew install python3), but numpy and scipy don't have binary wheels for 3.6 yet so it's trying to compile from source and failing."