Treat eval_gemfile path as relative to the root gemfile

It was previously treated as relative to the working directory, so one couldn't e.g. specify `eval_gemfile 'Gemfile.devtools'` and `chdir somedir/ && bundle exec something`.

I just fixed my spec dependencies, and will have a spec added in a bit. This should still be compatible with `eval_gemfile '/absolute/path/to/Gemfile'`, thanks to Pathname#join.