Use /dev/urandom, not /dev/random

Because /dev/random blocks which is super irritating (tokens take >30
seconds to generate on my machine)

Read this: http://www.2uo.de/myths-about-urandom/

If anyone can authoratitively disagree, please feel free to comment and
close this PR :)
Merge pull request #262 from alphagov/use-urandom-not-random

Use /dev/urandom, not /dev/random