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)