EE files are excluded from backtraces

As you can see in https://gitlab.com/gitlab-org/gitlab-ee/issues/5203, the backtrace doesn't include any EE code, even though this is an EE-specific problem.

I think https://github.com/rails/rails/blob/4-2-stable/railties/lib/rails/backtrace_cleaner.rb#L5 is the problem here, and we should define our own silencer.

To reproduce:

$ bundle exec rails r 'begin; License.plans_with_feature(:geo); rescue => e; pp Rails.backtrace_cleaner.clean(e.backtrace); end'
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