RSpec3

This works with both RSpec2 and RSpec3. To test that, use `RSPEC2` environment variable.

```
bundle install -j8
bundle exec rspec # all tests are passed

RSPEC2=1 bundle update -j8
RSPEC2=1 bundle exec rspec # all tests are passed
```

- Use yielded `Example` object.
- Test with Rails4.1.6.
- Remove all warnings.