Support for macOS

Adds support for macOS

Support for Ubuntu 16.04

Merge pull request #5 from clupprich/matrix-builds

* fix dependencies for Ubuntu 16.04
* run tests in a matrix for OS version,

Support skipping building Ruby when available in cache

Adds the cache-available input that will skip building the specified version of Ruby from source if it's already available in the cache.

See the README for an example.

First working version

Allows you to install any Ruby version that's supported by ruby-build. See the list here: https://github.com/rbenv/ruby-build/tree/master/share/ruby-build