Move Rails/RelativeDateConstant to basic cops

The reason for this rule is similar to dynamic factories: we want
dynamic dates to be evaluted relative to the current time, and not
relative to the moment the application was loaded.

.rubocop_basic.yml

@@ -59,6 +59,9 @@ Rails/ApplicationRecord:
 Rails/HttpPositionalArguments:
   Enabled: true
 
+Rails/RelativeDateConstant:
+  Enabled: true
+
 RSpec/NotToNot:
   Enabled: true