Update Exceptional Rake integration

The previous version of your Rake integration didn't work with the latest Rake. 

Overwriting ***display_error_message*** should be enough for an Exceptional Integration.

This isn't tested with older rake versions though. See https://github.com/honeybadger-io/honeybadger-ruby/blob/master/lib/honeybadger/rake_handler.rb for an implementation which works with older rake versions.