Update plugin schema version

[jmauzyk]

Existing Back In Stock installations need the plugin's schema version to be changed in order to trigger the migrations to be run. Per the Craft Migrations docs:

If this is a plugin migration, increase your plugin’s schema version, so Craft knows to check for new plugin migrations as people update to your new version.

Without this, the migration will not be run and will result in the errors mentioned in #19 and #20.