Tweak Rails::Generators.hidden_namespaces so no RSpec generators are hidden...

... from command line help. When running "rails g", command line help now shows all RSpec generators:

Rspec:
  rspec:controller
  rspec:feature
  rspec:helper
  rspec:install
  rspec:integration
  rspec:job
  rspec:mailer
  rspec:model
  rspec:observer
  rspec:scaffold
  rspec:view