only use the master db for addon_last_updated

https://bugzilla.mozilla.org/show_bug.cgi?id=951703

This might fix the `IntegrityError` from `addon_last_updated` in [mat's gist](https://gist.github.com/diox/502a20a58d156e5d375a).

For some reason django thinks it needs to do an insert but it should be doing an update. After a talking to @diox it seems like it might have something to do with mysql master/slave. Adding the `write` decorator will hopefully fix that.