Reduce the dependencies of the chatops controller

When looking at options to mitigate
https://github.com/github/github/issues/174690, my initial approach
there failed because we can't drop some Rails components because we use
gems that express all of Rails as a dependency instead of the parts they
need.

The chatops controller is one of those. It looks like though it only
needs ActiveSupport & ActionPack so I reduced the dependencies here to
just those parts.

chatops-controller.gemspec

@@ -17,7 +17,8 @@ Gem::Specification.new do |s|
   s.files = Dir["{app,config,db,lib}/**/*", "README.md"]
   s.test_files = Dir["spec/**/*"]
 
-  s.add_dependency "rails", ">= 6.0"
+  s.add_dependency "actionpack", ">= 6.0"
+  s.add_dependency "activesupport", ">= 6.0"
 
   s.add_development_dependency "rspec-rails", "~> 3"
   s.add_development_dependency "pry", "~> 0"