Reference classes instead of using strings
Reference classes instead of using strings

This fixes the Rails 5 deprecation warning:

```
DEPRECATION WARNING: Passing strings or symbols to the middleware builder is deprecated, please change
them to actual class references.  For example:

  "Bugsnag::Rack" => Bugsnag::Rack
```
bugsnag/bugsnag-ruby,bugsnag/bugsnag-ruby,bugsnag/bugsnag-ruby,bugsnag/bugsnag-ruby