Request for public keys only if LDAP attribute is set (#5816) (#5819)

* Update go-ldap dependency

* Request for public keys only if attribute is set

vendor/gopkg.in/ldap.v2/atomic_value.go

@@ -0,0 +1,13 @@
+// +build go1.4
+
+package ldap
+
+import (
+	"sync/atomic"
+)
+
+// For compilers that support it, we just use the underlying sync/atomic.Value
+// type.
+type atomicValue struct {
+	atomic.Value
+}