Rails 4.0 compatiblity

• Drops support for Rails 3.0 (There are 8 currently 8 failings tests and Rails 3.0 is no longer maintained)
• Bump supported activerecord version to >= 3.1
• Explicitly tests all currently supported Rails versions (3.1, 3.2 and 4.0)
• Fixes tests to work without deprecated active_record finders