Loosen minitest dependency.  Add Gemfile for Rails 4.1.0.beta1

Remove the branches restriction from Travis config.

Get Rubinius running again

Add Rubinius gems
Add rubinius-developer_tools so Coveralls doesn't error.  Explicitly add 'json' gem to Rails 4.0.x case, as it isn't explicity required (but is used) by Rails 4.0.x
Merge pull request #729 from petergoldstein/feature/rails_4_1

Travis - Add Rails 4.1.0.beta1, get Rubinius running
Update Gemfiles

* Only include simplecov gem on MRI 2.1
* Only include coveralls gem on MRI 2.1
* Remove rubinius-developer_tools, which is no longer needed since we
are not running coveralls on rubinius
* Do not try to require coveralls on rubinius (`platform :mri_21`
includes rbx, so we need to check if Rubinius is defined)
Fix test_compile_error tests for different platforms