Fix godeps

Signed-off-by: Olivier Gambier <olivier@docker.com>

vendor/google.golang.org/grpc/credentials/credentials.go

@@ -35,7 +35,7 @@
 // which encapsulate all the state needed by a client to authenticate with a
 // server and make various assertions, e.g., about the client's identity, role,
 // or whether it is authorized to make a particular call.
-package credentials // import "google.golang.org/grpc/credentials"
+package credentials
 
 import (
 	"crypto/tls"