Enable HWE support

The GOV.UK mirrors run Ubuntu 12.04 (Precise), for which the kernel has now EOL'd. Ubuntu's [workaround](https://wiki.ubuntu.com/1204_HWE_EOL) for this allows hardware support to be brought forward from a later release (in this case, 14.04 (Trusty)), and thus make the installed kernel supported for a longer period of time.

At some stage, we might want to look at upgrading these to Trusty.