Modify dead repositoty to living repository 

ppa:cassou/emacs is dead repository.
ref: https://github.com/cask/cask/issues/172#issuecomment-32716449
ref: https://github.com/rolandwalker/emacs-travis/commit/91d98ca0688313d6118ea5646f962de53204481e

I think ppa:ubuntu-elisp/ppa is good repository.