Move the `cache: directories: "$HOME/.ccache"` key from the top of the
file into each specific job, otherwise it does not seem to have an
effect as a global option.
Use a matrix of 4 jobs, linux (ubuntu trusty), mingw i686, mingw x86-64
and mac.
Automatically detect ccache in the cmake config and cache the
`~/.ccache` directory for faster rebuilds.
Instructions for ccache in cmake are here:
https://stackoverflow.com/a/24305849
Change build status badge in README.md to Travis.
Rafael Kitover