Update Berksfile.lock to be in sync with the latest Berksfile.

Berksfile specifies the dhcp cookbook from git://github.com/spheromak/dhcp-cook.git,
while the checked in Berksfile.lock pointed to git://github.com/fnichol/dhcp-cookbook.git,
which does not work wihout `data_bags/dhcp/default.json`, which was removed from
this repo on 17 April.