Log Message:
Update to 1.6.0:

v1.6.0
======

* bail out early if the scm is missing

  this brings issues with git tarballs and
  older devpi-client releases to light,
  before we would let the setup stay at version 0.0,
  now there is a ValueError

* propperly raise errors on write_to missuse (thanks Te-jé Rodgers)