Remove deprcated use of InstanceMethods in Prawnto::ActionController

Updated deprecated usage of ActiveSupport::Concern gets rid of the following warnings
```
DEPRECATION WARNING: The InstanceMethods module inside ActiveSupport::Concern will be no longer included automatically. Please define instance methods directly in Prawnto::ActionController instead. (called from include at death_star/vendor/plugins/prawnto/lib/prawnto.rb:24)
DEPRECATION WARNING: The InstanceMethods module inside ActiveSupport::Concern will be no longer included automatically. Please define instance methods directly in Prawnto::ActionController instead. (called from include at death_star/vendor/plugins/prawnto/init.rb:10)
```