Tiny refactoring of setting user's uuid key

Remove tiny duplication of the functionality of Exercism.uuid
Explicitly return `true` from before_create callback as per:

http://stackoverflow.com/questions/4735141/rails-3-do-i-need-to-give-return-true-in-a-before-save-callback-for-an-object
and
http://blog.teksol.info/2010/09/28/unintented-consequences-the-pitfalls-of-activerecord-callbacks