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: I2f9a9c2be4c26d376c381df9cf96ee5461015b5b
Signed-off-by: Sarah Liske <sarah.liske@polybeacon.com>