The cmake download was failing, so we were relying on the old cmake,
which I broke recently (in 7ae96c710b).
Now the download should work again.
Also fixed warning that the requested OSX image no longer exists and
that it is automatically substituted for xcode 7.3 image.
Adding several improvements, such as:
- making use of the container based infrastructure
- using ccache for faster building
- builds for linux, osx
- builds for verious gcc, clang versions
- recent cmake installation
