add timestamps to group_categories table

test plan:
- run migrations
- look at a group category in the console
- it should have a created_at and updated_at field

Change-Id: Ie09dce263e4dcbd2bc34b4d296fab190a21f7c2d
Reviewed-on: https://gerrit.instructure.com/64775
Tested-by: Jenkins
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>
QA-Review: Simon Williams <simon@instructure.com>

db/migrate/20151006220031_add_timestamps_to_group_categories.rb

@@ -0,0 +1,9 @@
+class AddTimestampsToGroupCategories < ActiveRecord::Migration
+  tag :predeploy
+
+  def change
+    change_table(:group_categories) do |t|
+      t.timestamps
+    end
+  end
+end