Ignore `./vendor` content in travis git-validation
Ignore `./vendor` content in travis git-validation

Ignores vendor content for `make dco` use of git-validation tool.

Signed-off-by: Phil Estes <dbac1f450dc8326caa7ba104b384a0eca642250c@linux.vnet.ibm.com>

kunalkushwaha/containerd,lichao-mobanche/containerd,miaoyq/containerd,chanezon/containerd,containerd/containerd,containerd/containerd,mikebrow/containerd,AkihiroSuda/containerd,crosbymichael/containerd,hqhq/containerd,yuexiao-wang/containerd,miaoyq/containerd,kunalkushwaha/containerd,crosbymichael/containerd,docker/containerd,Wolphin-project/containerd,thaJeztah/containerd,dmcgowan/containerd,estesp/containerd,vdemeester/containerd,dmcgowan/containerd,vdemeester/containerd,xiekeyang/containerd,rajasec/containerd,estesp/containerd,thaJeztah/containerd,mikebrow/containerd