Fix rr deprecation warning when using it together with rspec

This commit fixes the following error:

    DEPRECATION: RSpec::Core::Configuration#backtrace_clean_patterns is
   deprecated. Use RSpec::Core::Configuration#backtrace_exclusion_patterns
   instead.

...that appears when using rspec + rr. Since a while there's no need of using
the `conf.with_mock :rr` in Rspec conf. See more info [here][1].

[1]: https://github.com/rr/rr/pull/37#issuecomment-22833629