Get rid of a Rails 5 deprecation warning in...

Get rid of a Rails 5 deprecation warning in db/migrate/20170425112128_create_pipeline_schedules_table.rb
Signed-off-by: Rémy Coutable <remy@rymai.me>

db/migrate/20170425112128_create_pipeline_schedules_table.rb

@@ -17,7 +17,7 @@ class CreatePipelineSchedulesTable < ActiveRecord::Migration
       t.boolean :active, default: true
       t.datetime :deleted_at
 
-      t.timestamps
+      t.timestamps null: true
     end
 
     add_index(:ci_pipeline_schedules, :project_id)