switch to vndr

vndr is almost exactly the same as our old good hack/vendor.sh. Except
it's cleaner and it allows to re-vendor just one dependency if needed
(which we do a lot for containers/image).

Signed-off-by: Antonio Murdaca <runcom@redhat.com>

vendor/github.com/stretchr/testify/assert/assertion_forward.go.tmpl

@@ -1,4 +0,0 @@
-{{.CommentWithoutT "a"}}
-func (a *Assertions) {{.DocInfo.Name}}({{.Params}}) bool {
-	return {{.DocInfo.Name}}(a.t, {{.ForwardedParams}})
-}