Updating RSpec to v3

RSpec 3 has been released so it's time to update the specs to stay
up-to-date. The present commit changes the following things:

Replacements

* `obj.stub(:method)` with `allow(obj).to receive(:method)`
* `obj.stub(method: x)` with `allow(obj).to receive_messages(method: x)`
* `be_false` with `be false` or `be_falsey` depending on expectation
* `be_true` with `be true` or `be_truthy` depending on expectation

Dependencies

* json_spec is not a dependency anymore
* rspec is now '~> 3.0'
* error when loading `html/document` was fixed by requiring `action_view`

(Ref #152, Fix #164)