Add convert a missed timestamp column to timestamp with tz

db/migrate/20190505111117_convert_slots_ends_at_to_timestamp_with_tz.rb

@@ -0,0 +1,7 @@
+class ConvertSlotsEndsAtToTimestampWithTz < ActiveRecord::Migration[5.2]
+  def up
+    transaction do
+      execute "ALTER TABLE slots ALTER ends_at TYPE timestamptz USING ends_at AT TIME ZONE 'UTC';"
+    end
+  end
+end