Amend exec timeout from 300 to 600

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.
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.