Use the corect sensor-kind values for GretagMacbeth sensors

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1000910
libcolorhug: Handle failure to initialise GUsb in self-tests

This can fail, and does in my VMs and on the Ubuntu autopkgtest infrastructure.
Since there are other tests of libcolorhug that can usefully be run, handle this
like a failure to find a ColorHug on the USB bus, which is already non-fatal.
libcolord: Do not use G_GNUC_WARN_UNUSED_RESULT when uninhibiting

Clients don't need to know this has failed if a device was never inhibited in
the first place.
Release colord 1.0.6