Default LDAP server port should be 389

See https://github.com/rabbitmq/rabbitmq-auth-backend-ldap/issues/63#issuecomment-284680951.

It was changed to 3890 in 2461f04795
by mistake. We don't need to do that since the test suite reconfigures
the plugin to use 3890 anyway.

Fixes #64.

deps/rabbitmq_auth_backend_ldap/Makefile

@@ -17,7 +17,7 @@ define PROJECT_ENV
 	    {use_ssl,               false},
 	    {use_starttls,          false},
 	    {ssl_options,           []},
-	    {port,                  3890},
+	    {port,                  389},
 	    {timeout,               infinity},
 	    {log,                   false},
 	    {pool_size,             64},