Add OSX shards.

[jsirois]

Fixes #479

[Eric-Arellano]

Great! Glad to see the use of YAML variables.

As a separate issue, we might want to consider adding tests for Python 3.7. Currently Travis doesn’t natively support it though, so it’s very hacky to get support and adds about 3 minutes to CI to download the interpreter every time. travis-ci/travis-ci#9815 Probably still worth it? Do we have any PEX users using Python 3.7 that we know of? Foursquare temporarily used it but had to roll back to 3.6 due to issues with installing on Linux.

[jsirois]

I've filed #566 for 3.7 support. That will now be straightforward to add - and if I switch the linux shards to using pyenv, it will be at ~ no time cost since we're caching the pyenv already.