Use Rails.backtrace_cleaner.clean

The `filter` alias for `clean` was added in Rails 4.0.0.

Relevant commit: https://github.com/rails/rails/commit/d91bee80beae4e92b2b015599771310cebc3a143
Merge pull request #1 from gabebw/gbw-rails-3-compatibility

Use Rails.backtrace_cleaner.clean