Update PBR requirement to >=0.11
Update PBR requirement to >=0.11

0.11 has a fix for an alembic migration problem that has plagued
Gertty early adopters.  1.0 is out as well and will work, so it
is safe to set the semver cap at 2.0.

Change-Id: I5426fabcfa79e32a9bc1c2c91f10d4f20ff7b913

stackforge/gertty,openstack/gertty,aspiers/gertty