Cirrus CI recently started supporting Mac OS builds. This PR adds a CI task to test Puppeteer on MacOS.
It uses Node 8 since only [Node 6 and Node 8 are LTS](https://github.com/nodejs/Release#release-schedule) so `brew` only has receipts for those two TLS versions besides the latest one.
Cirrus CI got some optimizations for containers based of `microsoft/windowsservercore:latest`.
Now startup time for windows builds is around 1:30 seconds instead of around 4 minutes.
to: @aslushnikov
This patch adds experimental support for Cirrus CI builds.
Cirrus CI has both Windows and Linux, uses google cloud and runs builds
in Docker containers.
This seems to be a promising combination for our needs.
