Improve Travis CI configuration

Use the ubuntu-elisp PPA for nightly snapshot builds, as Cassou does not provide snapshot builds anymore, and simplify the installation of Emacs.
Merge pull request #93 from lunaryorn/patch-1

Improve Travis CI configuration
Alias -tail to nthcdr
Add -iteratefn
Reorder the partition examples to follow more logical order, add tests
Add -prodfn