Fix EE ActiveRecord::Migration deprecations

This fixes two cases of EE specific code extending
ActiveRecord::Migration, instead of extending
ActiveRecord::Migration[4.2].

ee/db/migrate/20181114163403_add_epics_sort_to_user_preference.rb

-class AddEpicsSortToUserPreference < ActiveRecord::Migration
+class AddEpicsSortToUserPreference < ActiveRecord::Migration[4.2]
   include Gitlab::Database::MigrationHelpers
 
   DOWNTIME = false

ee/db/post_migrate/20181116100917_sanitize_tracing_external_url.rb

@@ -3,7 +3,7 @@
 # See http://doc.gitlab.com/ce/development/migration_style_guide.html
 # for more information on how to write migrations for GitLab.
 
-class SanitizeTracingExternalUrl < ActiveRecord::Migration
+class SanitizeTracingExternalUrl < ActiveRecord::Migration[4.2]
   include Gitlab::Database::MigrationHelpers
 
   # Set this constant to true if this migration requires downtime.