Fix regexp warnings on 1.8.7.

```
./spec/rspec/mocks/argument_matchers_spec.rb:277: warning: regexp has invalid interval
./spec/rspec/mocks/argument_matchers_spec.rb:277: warning: regexp has `}' without escape
./spec/rspec/mocks/argument_matchers_spec.rb:284: warning: regexp has invalid interval
./spec/rspec/mocks/argument_matchers_spec.rb:284: warning: regexp has `}' without escape
```