Add the version constraint for actionpack

#38 introduced
the check whether the route comes from Rails with `route.app.engine?`.

However, the method `engine?` was added in this PR,
and this is available since Rails 5.2.

rails/rails@b50e88e

So, adding the version constraint might make sense.

Note Rails 5.2 reached EOL, which means patches against
severe security issues will not be available, so I added `>= 6.0.0`
instead of `>= 5.2.0`.

rspec-openapi.gemspec

@@ -23,6 +23,6 @@ Gem::Specification.new do |spec|
   spec.executables   = spec.files.grep(%r{^exe/}) { |f| File.basename(f) }
   spec.require_paths = ['lib']
 
-  spec.add_dependency 'actionpack'
+  spec.add_dependency 'actionpack', '>= 6.0.0'
   spec.add_dependency 'rspec'
 end