Travis: Use Anil's PPA

See: http://anil.recoil.org/2013/09/30/travis-and-ocaml.html
Travis: Execute 'arakoon.native --run-all-tests'

Since we no longer need to build OCaml from source, we might have time
to run some real tests now within a TravisCI job slot.
Merge pull request #211 from Incubaid/travis-ppa

Use a PPA iso building OCaml & opam from source