[#73607174] Enable automatic reboot after unattended-upgrades

...so that we don't need to manually reboot the machine after critical
security updates are applied.

Install the `update-notifier-common` package as it's required for the
automatic reboots to work. `update-notifier-common` is responsible for
creating the `/var/run/reboot-required` file.

If a reboot is required, it will happen immediately after
`unattended-upgrades` has run at approximately 06:25 in the morning.

As far as I can tell, no jobs run on the CI slaves at that time:

__Slave 1:__
![slave1](https://cloud.githubusercontent.com/assets/4348848/5220455/3518cede-765d-11e4-9d8e-19eea1ecb743.png)

__Slave 2:__
![slave2](https://cloud.githubusercontent.com/assets/4348848/5220456/351bbf54-765d-11e4-891b-02c82a8b6cd3.png)

__Slave 3:__
![slave3](https://cloud.githubusercontent.com/assets/4348848/5220458/3521b54e-765d-11e4-9728-454840b8aa71.png)

__Slave 4:__
![slave4](https://cloud.githubusercontent.com/assets/4348848/5220457/351f3238-765d-11e4-88b4-f1dc403e136c.png)

A reboot would affect the transition-logs-1 machine, though reboots are usually infrequent. /cc @jennyd @jamiecobbett