Switch to using the dep tool and update all the dependencies

vendor/golang.org/x/crypto/ssh/doc.go

@@ -14,5 +14,8 @@ others.
 References:
   [PROTOCOL.certkeys]: http://cvsweb.openbsd.org/cgi-bin/cvsweb/src/usr.bin/ssh/PROTOCOL.certkeys?rev=HEAD
   [SSH-PARAMETERS]:    http://www.iana.org/assignments/ssh-parameters/ssh-parameters.xml#ssh-parameters-1
+
+This package does not fall under the stability promise of the Go language itself,
+so its API may be changed when pressing needs arise.
 */
-package ssh
+package ssh // import "golang.org/x/crypto/ssh"