Pin pbr version to <0.7.0

The reason for pinning to <0.7.0 was PBR bumped the pip requirement to 1.4+.
Currently we don't want to force people to upgrade pip to run the latest
version of PBR.

Change-Id: I43e696565c622659174088ea5699ebfd2a45e558
Signed-off-by: Sarah Liske <sarah.liske@polybeacon.com>