Make -U work for FreeBSD

When using http://files.wunki.org/freebsd-9.2-amd64-wunki.box or
http://files.wunki.org/freebsd-10.0-amd64-wunki.box with Vagrant,
bootstrap would succeed, but a number of states would fail without
a package upgrade prior to running highstate. The -U switch currently
does not trigger an upgrade when bootstraping FreeBSD. This patch adds
a package upgrade to the end of the FreeBSD dependency install
function.
Merge pull request #518 from eshagh/bsd-update

Make -U work for FreeBSD