Fixed newrelic_ignore problem

When I use this gem with RailsAdmin gem, I get the following error.

```
undefined local variable or method `newrelic_ignore' for RailsAdmin::ApplicationController:Class (NameError)
```

Though this problem occurs because RailsAdmin uses `newrelic_ignore` method internally, I think that the same problems will occur when it is used in other places.

So, I suppose this problem should be fixed.