Fix named group support in Rails 5

Rails 5 made `belongs_to` associations required by default. When using named groups, the group association is expected to be empty. This fixes the issue in a Rails 4 compatible way by adding `required: false`

Fixes #65

lib/groupify/adapter/active_record/group_membership.rb

@@ -14,7 +14,7 @@ module GroupMembership
 
       included do
         belongs_to :member, polymorphic: true
-        belongs_to :group, polymorphic: true
+        belongs_to :group, polymorphic: true, required: false
       end
 
       def membership_type=(membership_type)