Add CI to project

I got [your comment](https://github.com/iamvery/delayed_job_recurring/commit/e6f9d410ceddc83383dbe2217b63218a050918b4#commitcomment-8622903) and realized that I mistakenly thought the code worked fine without the line. This was because I failed to tear down my test database before running the test suite again. Here's some continuous integration to help catch these things for us! I'll rework the delayed job 3.x compatibility with test coverage :smile: