Switch from using passphrase_entropy to a ruby port of zxcvbn.

Uses `zxcvbn-ruby` which in initial tests is at least 30 time faster than `passphrase_entropy`.
The `passphrase_entropy` implementation seems rather basic, where `zxcvbn-ruby` also has the added security of scoring 0 to known 85094 common passwords.

More info: https://tech.dropbox.com/2012/04/zxcvbn-realistic-password-strength-estimation/

Might want to use an alphagov fork of [devise_zxcvbn](http://github.com/bitzesty/devise_zxcvbn)