allow git+ssh as a scheme (#72)

It turns out that sometimes this is a valid secure scheme to use. It should be allowed.

gbvendor/repo.go

diff --git a/gbvendor/repo.go b/gbvendor/repo.go
index 82f4934..6121f25 100644
--- a/gbvendor/repo.go
+++ b/gbvendor/repo.go
@@ -273,7 +273,7 @@
 		url.Scheme = scheme
 
 		switch url.Scheme {
-		case "https", "ssh":
+		case "git+ssh", "https", "ssh":
 			if err := vcs(&url); err == nil {
 				return url.String(), nil
 			}