Add Rails/WhereNotWithMultipleConditions rule

This rule was introduced in rubocop-rails 2.17.0.

We don't use `where.not` with multiple conditions anywhere, but if we
did, it would indeed be very confusing, so we're adding a rule to avoid
that scenario.

.rubocop.yml

@@ -455,6 +455,9 @@ Rails/WhereEquals:
 Rails/WhereNot:
   Enabled: true
 
+Rails/WhereNotWithMultipleConditions:
+  Enabled: true
+
 RSpec/AroundBlock:
   Enabled: true