Write rubinius specs for Module#thunk_method

* https://github.com/rubinius/rubinius/pull/2505#issuecomment-21705864
* https://github.com/rubinius/rubinius/pull/2505#pullrequestreviewcomment-5479225
Write rubinius specs for Module#set_visibility
Write rubinius specs for Module#set_class_visibility
Privatize a rubinius method Module#set_class_visibility

https://github.com/rubinius/rubinius/pull/2505#issuecomment-21666613
Merge pull request #2505 from kachick/clean-module-nonruby_methods

Remove/Hide some non-Ruby methods from Module