Add void column to spree license keys.

There are cases where one would like to void a license key without
having to either destroy it or artificially add an inventory unit to it.
This column makes that possible. It is false by default so if never set,
it does not change anything.

Change-Id: Ic6d35553001d42a42a847959c44795c5d8a04ac1
Add memo column to spree license keys.

Having a text field column on each license key is really useful for
customer support, etc.

Change-Id: Icf0b7d4327a6eeafc48e7c0eaf96f2acf0378c1d
Explicitly update timestamps when assigning license keys.

update_all is used to assign an inventory unit to a set of license keys,
but this does not instantiate models and so it does not update
timestamps. This change forces the update to set timestamps when
assigning keys.

Change-Id: I8dd497dc9c59415561334920aee752ddc7c45fc5