Enable UFW after setting firewall rules

On fresh installs of Debian 7.6, the current order of steps will lock you
out of SSH. This will enable UFW after creating rules for http, https, ssh,
and DNS. Fix comes from @Debugreality in issue #303:

https://github.com/al3x/sovereign/issues/303
Merge pull request #48 from apsanz/master

Enable UFW after setting firewall rules