Added code coverage badge.

Hi, I've added a coverage % badge that uses https://coveralls.io stats:

[![Coverage Status](https://coveralls.io/repos/lemurheavy/delayed_job/badge.png?branch=master)][coveralls]

[coveralls]: https://coveralls.io/r/lemurheavy/delayed_job


The badge for ```collectiveidea/delayed_job``` is ready to go, it will just need to be updated from ```lemurheavy/delayed_job``` once merged in.

Thanks!