Fix vendor validation
Fix vendor validation

The dep-validate target appears to never get invoked. In the CircleCI
build environment, "master" points to the commit under test. The
circle.yml fragment needs to compare again "origin/master" instead.

Signed-off-by: Aaron Lehmann <8ecfc6017a87905413dcd7d63696a2a4c351b604@docker.com>

miminar/distribution,konnase/distribution,jwhonce/distribution,sergeyfd/distribution,huamai-io/distribution,miminar/distribution,harrisonfeng/distribution,docker/distribution,jzwlqx/distribution,vdemeester/distribution,estesp/distribution,duglin/distribution,manoj535/distribution,konstruktoid/distribution,cezarsa/distribution,thaJeztah/distribution,sergeyfd/distribution,konstruktoid/distribution,yuwaMSFT2/distribution,harrisonfeng/distribution,shyr/distribution,manoj535/distribution,mikebrow/distribution,dmcgowan/distribution,jzwlqx/distribution,thaJeztah/distribution,cezarsa/distribution,estesp/distribution,konnase/distribution,shyr/distribution,docker/distribution,dmcgowan/distribution,mikebrow/distribution,sadlil/distribution,huamai-io/distribution,yuwaMSFT2/distribution,jwhonce/distribution,duglin/distribution,vdemeester/distribution,sadlil/distribution