Closes #17: replace cl-eval-when with eval-when-compile

* slime.el (require 'arc-mode): Replace cl-eval-when with
eval-when-compile, broke in emacs 24.2. (Closes #17).
Improve Travis CI coverage

* .travis.yml: Cleaner, more LISP combinations, but test only emacs 23 and
emacs 24 on debian.