[python-committers] macOS builds conditionally turned on for Travis

I have turned on macOS builds on Travis for all active branches, but for now the builds are allowed to fail. The latter detail is so that we can be sure that the tests are stable on Travis before blocking on it as well as see if the added build time is acceptable for blocking a PR on. If the tests end up being stable for at least a week and the added time is deemed acceptable then we can make the macOS builds required for Travis to be considered passing.

I'm using https://github.com/python/core-workflow/issues/91 to track the requirements discussion.