Can not `lock!`an ActiveRecord instance

Add this to `spec/models/post_spec.rb` and it will fail:

```
describe Post do
  describe "Reloading a record" do
  ....
    context "while locking" do
      it "does not throw an error" do
        expect(lambda { subject.lock! }).to_not raise_error
      end
    end
```

I have a PR #54 that fixes this but also wanted to leave the issue here formally.
