Fix column differences in `db/schema.rb`

project_custom_attributes and group_custom_attributes used datetime
instead of datetime_with_timezone column type in database schema. But
actually migration defines `t.timestamps_with_timezone`.

db/schema.rb

@@ -943,8 +943,8 @@ ActiveRecord::Schema.define(version: 20180906101639) do
   add_index "gpg_signatures", ["project_id"], name: "index_gpg_signatures_on_project_id", using: :btree
 
   create_table "group_custom_attributes", force: :cascade do |t|
-    t.datetime "created_at", null: false
-    t.datetime "updated_at", null: false
+    t.datetime_with_timezone "created_at", null: false
+    t.datetime_with_timezone "updated_at", null: false
     t.integer "group_id", null: false
     t.string "key", null: false
     t.string "value", null: false
@@ -1550,8 +1550,8 @@ ActiveRecord::Schema.define(version: 20180906101639) do
   add_index "project_ci_cd_settings", ["project_id"], name: "index_project_ci_cd_settings_on_project_id", unique: true, using: :btree
 
   create_table "project_custom_attributes", force: :cascade do |t|
-    t.datetime "created_at", null: false
-    t.datetime "updated_at", null: false
+    t.datetime_with_timezone "created_at", null: false
+    t.datetime_with_timezone "updated_at", null: false
     t.integer "project_id", null: false
     t.string "key", null: false
     t.string "value", null: false