Upgrade to rspec 3.0.0.rc1

Since rspec-rails 3.0.0.rc1, spec types are no longer inferred by
location, they instead need to be explicitly tagged.
https://github.com/rspec/rspec-rails/blob/master/Changelog.md#300rc1--2014-05-18

stub_model has moved to rspec-activemodel-mocks gem.

fuubar 2.0.0.beta2 doesn't work with rspec 3.0.0.rc1 and disable it
temporarily.