Rails check is insufficient

[robertino]

We have a Sinatra app that uses actionpack gem.
actionpack in turn has a dependency on rails-html-sanitizer gem.
rails-html-sanitizer defines Rails module.
airbrake has a few occurences where a check of defined?(Rails) is performed.
This leads to uninitialized constant Rails::Engine error down the line, as airbrake/rails/app.rb gets included.
The solution to the problem would be a better check for actual existence of Rails framework rather than simple defined?(Rails), otherwise any Rails module/class reference without actual use of rails gem/framework will cause issues with Airbrake (airbrake gem).