Generate .travis.yml

We want to generate this file because:

* We're using Travis on a lot of projects
* It requires configuration to run builds and notify us
* We can skip some boilerplate by starting out with a sane config

The generated config makes sense because:

* It can run Ruby builds out of the box with our Ruby version
* It reduces noise for Campfire notifications
* It reduces volume in the queue by avoiding unnecessary branch builds

This also refactors handling of Ruby versions to move interpolation into 
templates where possible.