Compatibility with Mongoid 4.0 and DJ 4.0.

Since Mongoid 4.x hasn't been released, you can test this by adding the following to Gemfile above `gemspec`:

```
gem "delayed_job", "~> 4.0"
gem "mongoid", github: "mongoid/mongoid"
```

We can lock both `< 5` if you prefer, lmk.

Bonus: Updated CHANGELOG with release notes for 2.0 and next release.