Fixed NameError

The ROTP was being referenced without properly load. The require is
enough to fix the issue.

Backtrace:

> NameError - uninitialized constant ROTP:
  activesupport (4.1.4) lib/active_support/dependencies.rb:518:in `load_missing_constant'
  activesupport (4.1.4) lib/active_support/dependencies.rb:180:in `const_missing'
   () Users/paulo/.rvm/gems/ruby-2.1.2@auth/bundler/gems/CASino-a2ec29c1a104/app/processors/casino/processor_concern/two_factor_authenticators.rb:12:in `validate_one_time_password'
   () Users/paulo/.rvm/gems/ruby-2.1.2@auth/bundler/gems/CASino-a2ec29c1a104/app/processors/casino/second_factor_authentication_acceptor_processor.rb:24:in `process'
   () Users/paulo/.rvm/gems/ruby-2.1.2@auth/bundler/gems/CASino-a2ec29c1a104/app/controllers/casino/sessions_controller.rb:30:in `validate_otp'