add missing inventory_unit factory.
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
Replace update_all with update_attributes on each license key.

In the end it's too problematic to use an update_all here, since we need
callbacks to be run, etc. There are only ever a few license keys anyway
so this should not result in any serious performance issues.

Change-Id: Ib9e58103acc36cb41b332a9edadaf766a5fa9307
add .travis.yml.