Delayed job 3.x CI

:warning: This will need to be rebased once #3 and #4 get merged, but I wanted to get it out there for discussion sooner than later.

This changeset is a little larger than I would have liked:

1. Loosen version dependency for delayed job
1. Add travis config to integrate with delayed job 3.x and 4.x
1. Update test setup to be compatible with AR 3.x and 4.x

I'm not a huge fan of how big this diff is. I spent a little time trying to determine how to load the schema in a 3.x app. Rather than checking versions and going down that rabbit hole, I opted to remove the "skeleton" Rails app and only create the needed table. The required me to move around some `require`s as well...

What do you think? [:eyes:](https://travis-ci.org/iamvery/delayed_job_recurring/builds/41420756)