Add continuous test integration for OSX

[giampaolo]

Similarly to #646 but for OSX. Still have to investigate whether there are public free services providing this.

[giampaolo]

http://www.yegor256.com/2014/10/05/ten-hosted-continuous-integration-services.html
Apparently the only solution is https://hosted-ci.com/ but it's not free. https://ship.io/ does not seem to support OSX, only IOS.

[mrjefftang]

Travis CI supports it but they are full currently. You may want to get in contact with them for since psutil is an open source project.

[giampaolo]

from http://docs.travis-ci.com/user/multi-os/

We are at capacity with Mac workers in both .org and .com environments. Consequently, we are not
accepting requests for the multi-OS feature at this time. As more capacity is added, we will resume
accepting requests.

I'm gonna try to send them an email anyway.

[giampaolo]

See comments in: travis-ci/travis-ci#2312 (comment)

[giampaolo]

AFAIU OSX is not officially supported for language: python but different projects used a workaround consisting in using language: generic + os: osx then they install Python as part of the build installation process.
See for example this: https://github.com/pyca/cryptography/blob/master/.travis.yml (referecenced by urllib3/urllib3#392 (comment)).

[mrjefftang]

Figuring out the correct Travis settings will be a matter of trial and error as long as we can get it access to Travis's OSX builders.

[giampaolo]

Fixed by @mrjefftang. Closing.