Redirect to appropriate Migrations Guide version

Follow-up to 6eafc6e.

`migrations.html` was renamed to `active_record_migrations.html` in
Rails 4.2, but the v4.2 and v5.0 guides link to the old path.  Visiting
`migrations.html` in v4.2 or later, including stable, currently
redirects to the edge version of `active_record_migrations.html`, which
may go unnoticed and cause confusion.

This commit ensures `migrations.html` redirects to the appropriate
`active_record_migrations.html` for the v4.2 and later guides.

Author: jonathanhefner

config/nginx.conf

@@ -41,6 +41,7 @@ server {
 
     rewrite /contributing_to_rails.html /contributing_to_ruby_on_rails.html permanent;
     rewrite /contribute.html /contributing_to_ruby_on_rails.html permanent;
+    rewrite ^((?!/v[2-3]\.|/v4\.[0-1]).*)/migrations.html$ $1/active_record_migrations.html permanent;
     rewrite ^(.*)/constant_autoloading_and_reloading.html$ $1/autoloading_and_reloading_constants.html permanent;
 
     location ~ ^/v\d {