Add CI for Mac OS X

[robo9k]

Unfortunately Travis CI does not accept requests to enable OS X builds currently.

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.

If there is another free CI service to build on a Mac, we might give it a try.

[robo9k]

Looks like hosted-ci.com's free plan might work. I could not find any documentation, so I don't know whether it requires using an Xcode project.

[robo9k]

Circle CI has experimental support for OS X build machines and a free plan as well.

[robo9k]

I've signed up for CircleCI and they've enabled our GitHub organization (chip8-rust).
Currently there's a problem since the build seems to require an Xcode project/workspace, I'm chatting with their support.

[robo9k]

CircleCI requires an Xcode project/workspace and after asking their support:

[..] for our beta we are focusing on iOS projects. For other languages we just support a Linux environment.