Set RNG to /dev/urandom

This commit allows rng-tools to work in virtual machines, and precludes the error 'Cannot find a hardware RNG device to use' or the following in build.log:
````
Starting Hardware RNG entropy gatherer daemon: (failed).
invoke-rc.d: initscript rng-tools, action "start" failed.
````
Some more info here: https://bugs.launchpad.net/ubuntu/+source/gnupg/+bug/706011
Might want to run this by the security/audit people.