Use both cassou and ubuntu-elisp PPA

It looks like ~ubuntu-elisp and ~cassou provides different
versions (daily vs bimonthly) so it makes sense to try both.

See:
- https://github.com/cask/cask/issues/172
- https://launchpad.net/~ubuntu-elisp/+archive/ppa/
- https://launchpad.net/~cassou/+archive/emacs