Do not put timestamps at the bottom of a migration

`rails generate` puts timestamps at the bottom of a migration by default,
meaning we have to change every migration even if the migration is otherwise
generated perfectly.

Original discussion: thoughtbot/guides#159

Author: fktkt66551404RobertAlexander

style/rails/README.md

@@ -30,9 +30,7 @@ Migrations
 
 * Set an empty string as the default constraint for non-required string and text
   fields. [Example][default example].
-* List timestamps first when creating a new table. [Example][timestamps example].
 
-[timestamps example]: migration.rb
 [default example]: migration.rb#L6
 
 Routes