Use absolute_path of caller_locations to infer engine root

According to documentation `path` only returns file names. On MRI it's not the case but it's likely a bug in MRI. Rubinius tries to implement it according to documentation and Rails get broken on rbx ([relevant discussion](https://github.com/rubinius/rubinius/issues/3223)).

Affected versions of Rails are `>= 4.1.0`. Would you like me to submit a PR for 4.1 branch, too?