Loosen dependency on delayed job

* Interface is compatible with dj 3.0, so let's allow them to use it
* I'm not convinced the particular version of DJ AR is important, so
  remove this constraint
Add rake development dependency
Add Rakefile

* default task runs test suite
Add travis configuration

* Supporting a few different ruby versions
Cache bundler for travis

* http://docs.travis-ci.com/user/caching/#Bundler
Add travis badge to readme

[skip ci]
Merge pull request #3 from iamvery/dj3

delayed_job 3.x
Merge pull request #4 from iamvery/travis

Add CI to project