Fix import error for zuul version with docs

When using sphinx-build to generate documentation, it would fail to
properly find our version information.

Change-Id: Iee14aa76b69ca9398688345cb2dcd0db9ecdaedc
Signed-off-by: Paul Belanger <paul.belanger@polybeacon.com>
Reviewed-on: https://review.openstack.org/25524
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Monty Taylor <mordred@inaugust.com>
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Tested-by: Jenkins