Fix BacktraceSilencer#noise when multiple silencers are configured

The existing implementation of the `noise` method in `ActiveSupport::BacktraceSilencer` does not work correctly if more than one silencer is configured -- specifically, it will only return noise which is matched by all silencers (which would be an unusual requirement to say the least :smile:), so often returns a rather useless `[]`.

This PR modifies the implementation such that anything that has been matched by silencers is removed from the backtrace using `Array#-` (array difference), ie. returns those elements within a backtrace that have been matched by any silencer.

Shout if you would like this rebased on to or PRs created for any other branch(es).