Update .travis.yml

  * Test against Rubinius and better_errors 110/200
  * Do not allow jruby to fail any more. It's stable enough now.
  * Rubinius + Rails 4.2 still doesn't work, so allow it to fail.
:point_right: :point_right: :point_right:
Extract format method into a separate class
Add Pry formatter
Remove methods that haven't been used
Use plain formatter for better errors integration
Do not assume that Delegator is already 'require'd