Comply with pep440

pep440 indicates that development releases should mark themselves with
the devN specifier.

  http://www.python.org/dev/peps/pep-0440/#developmental-releases

Instead of using aN as we are currently doing for automatically
generated versions, we should use the pep440 compliant devN form.

Change-Id: Ieeb5149f8b013e76621112593e0a5c0a64817e1f