Require factory_girl 4.1.0 which supports unqualified generate

[#44034071]

The ability to call `generate` instead of `FactoyGirl.generate` in
factory definitions was added in 4.1.0.  Since the Gemfile.lock is not
committed (since this is a gem) use a version restriction in the Gemfile
to ensure at least 4.1.0 is installed.