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: I6d0ccd4586355e181ae0d1f483733a73b49e44aa
Signed-off-by: Sarah Liske <sarah.liske@polybeacon.com>