Add UUIDTools::UUID#next method to avoid a bug with shoulda-matchers

My RSpec suite randomly fail because of a bug between `shoulda-matchers` and `activeuuid `:

Indeed Shoulda relies on Type#next to randomly change an ID, but UUID#next was not implemented and returned UUID.to_s.next, which is or isn't an UUID !

I've patched UUID class to provide a random (but good) UUID on #next.