BackgrounDRb start error

Doesn't look like a 2.2.2 app. Seems you might be at 2.3.2. As you
have frozen Rails into vendor, you can back up and freeze a 2.2.2
version in its place (assuming you don't depend on any of the cool 2.3
features yet).

Yup the problem was because of vendored rails gem. You can pull again from backgroundrb repository and it should work alright.