[MRG] MacOS X testing improvements

Hanging multiprocessing tests are skipped on Mac OS X 10.9 in this PR.

`test_k_means_plus_plus_init_2_jobs` doesn't fail for me on 10.9, but I've added skipping code anyways because it was intended to be skipped before, and failures could be non-deterministic.

Also, I'm not sure customization hooks in `if_not_mac_os` (versions and message arguments) worth keeping. But at least it is better than old `_is_mac_os_version` function which had incorrect docstring.

Not Mac specific: `BaggingRegressor` was untested with n_jobs=-1 (most likely because of a type); this is also fixed.