Fallback when previous_value doesn't respond to #next.

For example, floating point attributes won't respond to #next, so in
this case fall back to adding 1. This is not perfect, but it seems
to rarely occur in practice since I wasn't able to find anyone else
complaining about it.

lib/shoulda/matchers/active_model/validate_uniqueness_of_matcher.rb

@@ -116,7 +116,11 @@ def validate_after_scope_change
               # Assume the scope is a foreign key if the field is nil
               previous_value ||= 0
 
-              next_value = previous_value.next
+              next_value = if previous_value.respond_to?(:next)
+                previous_value.next
+              else
+                previous_value + 1
+              end
 
               @subject.send("#{scope}=", next_value)