Amend exec timeout from 300 to 480

The default value of the `timeout` parameter used in the exec type in Puppet is
300secs. freshclam takes longer than 300secs to complete at times. If this
happens when a box is brought up, the provisioning may fail. Even though
freshclam will re-run after a while once the box has booted, it is far better to
ensure that we get a successful output from freshclam during provision, rather
than an error relating to it timing out.

This value has been set to 480, or eight minutes, given that the average run
time over three or so runs (as reported in freshclam.log) was about 6mins. Using
8mins gives us time in case a run takes longer, but not long enough that we'd
wait a long time for it to fail due to an error before reporting its failure.
Amend CHANGELOG

Release 0.0.2 was released earlier today with a change to qualify the `exec`
type in the `run_freshclam.pp` manifest. A later change was made to increase the
`timeout` parameter on the `exec` type to ensure that `freshclam` completes
successfully during a Puppet run without failing. Given that both of these
changes are related to the same issue, I have rolled the latter change in with
the former.
Merge pull request #3 from alphagov/increase-exec-timeout

Increase exec timeout