gemspec: Disallow 6.0.0.rc2

This version of Rails won't work with this Thredded release because
the work-around for its bug has been removed.

Refs rails/rails#36761

thredded.gemspec

@@ -29,7 +29,7 @@ Thredded works with SQLite, MySQL (v5.6.4+), and PostgreSQL. See the demo at htt
   s.add_dependency 'kaminari'
   s.add_dependency 'nokogiri'
   s.add_dependency 'pundit', '>= 1.1.0'
-  s.add_dependency 'rails', '>= 4.2.10'
+  s.add_dependency 'rails', '>= 4.2.10', '!= 6.0.0.rc2'
   s.add_dependency 'rb-gravatar'
 
   # post rendering