RSpec not working because of missing Module#dup

Basic RSpec file:
```ruby
describe Object do
  it "is true" do
    true.should be_true
  end
end
```
using
rspec (2.12.0)
rspec-core (2.12.2)
rspec-expectations (2.12.1)
rspec-mocks (2.12.1)

fails with
```ERROR 2702 , Module#dup (NotImplementedError)```.

@timfel, can you have a look at that?