Fix ReactOnRailsPro::Utils uninitialized constant error

After Phase 6 restructure, the Pro gem needs to be explicitly required
before checking if its constants are defined. Just checking if the gem
is available in Gem.loaded_specs doesn't actually load the gem's code.

Fixes rspec-package-tests failures.

Author: justin808

lib/react_on_rails/utils.rb

@@ -259,6 +259,7 @@ def self.react_on_rails_pro?
       @react_on_rails_pro = begin
         return false unless gem_available?("react_on_rails_pro")
 
+        require "react_on_rails_pro" unless defined?(ReactOnRailsPro)
         ReactOnRailsPro::Utils.validated_license_data!.present?
       end
     end