Display the epoch timestamp of the last head fetch

Hi,

I intend to update Ansible's homebrew library (https://github.com/ansible/ansible/blob/devel/library/packaging/homebrew) to support the 'cache_valid_time' concept as apt does (https://github.com/ansible/ansible/blob/devel/library/packaging/apt#L47).

This change allows Ansible to query homebrew as to the last time the index was updated.

Thanks.

Ash.