Update vendor

vendor/github.com/docker/libtrust/doc.go

@@ -0,0 +1,9 @@
+/*
+Package libtrust provides an interface for managing authentication and
+authorization using public key cryptography. Authentication is handled
+using the identity attached to the public key and verified through TLS
+x509 certificates, a key challenge, or signature. Authorization and
+access control is managed through a trust graph distributed between
+both remote trust servers and locally cached and managed data.
+*/
+package libtrust