Fix caller filter to ignore rubygems

This fixed rspec/rspec-core#1524.

I debated whether or not we should add code here to ignore rubygems but ultimately decided it was easier then auditing every `RSpec.deprecate` call to see if it could be triggered by a user `require` statement, so while this doesn't quite align with the `first_non_rspec_line` (since rubygems is non-rspec) it solves the problem and serves the purpose for which we created the caller filter.

Thoughts?

/cc @JonRowe @xaviershay