Auto-retry `bundle install` when Travis runs tests

`bundle install` doesn't always work -- sometimes it runs into errors
making HTTP requests, for whatever reason. This will cause Travis to
fail which is pretty annoying.

* Travis supplies an executable called `travis_retry` which will
  automatically retry the command up to 3 times before really failing.
  Tell Travis to use this when it runs `bundle install` before it
  runs tests.
* In spec_helper, we create a Rails app and use this within the
  test suite. This will also run `bundle install`. Unfortunately we
  can't use `travis_retry` for this as it's a function and is not
  available to us in Ruby-land, so use our own retry logic.