Use around instead of depricated before/after filter methods

Should fix the following warning seen when we run tests

    DEPRECATION WARNING: Filter object with #before and #after methods is deprecated. Define #around method instead. (called from block in <top (required)> at /Users/julian/.rvm/gems/ruby-2.0.0-p195/bundler/gems/audited-6cda413936b9/lib/audited/sweeper.rb:44)