Pin voluptuous <0.7.

* tools/pip-requires: A new voluptuous (0.7.0) was released today,
introducing backward-incompatible changes to schema objects and
breaking zuul's layout validation. This is a brown bag fix to keep
zuul functional until voluptuous 0.7 support is implemented.

Change-Id: I5818d3cd529e8682877b368edbc10280a5c167df
Reviewed-on: https://review.openstack.org/24454
Reviewed-by: Anita Kuno <akuno@lavabit.com>
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Approved: James E. Blair <corvus@inaugust.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins