Idris Support

[aatxe]

This PR should add initial support to Travis for Idris. The tests are based off the Rust tests, and I tried my best to model the install procedure after other community-maintained languages. The installation relies on first setting up stack (the Haskell build tool), and then using stack to install Idris. This process takes a very long time on Travis. The solution I came up with in my own tests (using the same commands in a language: generic config for Travis) was to cache stack to prevent recompiling Idris every time. With that cached, it only takes a minute or two. To that end, I've added stack and idris as possible cache settings to make configuration simpler. It's been a while since I wrote any Ruby. So, any feedback on this would be appreciated.

I've already seen the requirements for multiple supporters for community languages. To that end, I've recruited @david-christiansen and @jmitchell as my two additional supporters.