invalid reference notice

In line 330 of plugin version 1.5.2 a method result is passed by reference:

if ( wp_remote_retrieve_response_code( wp_remote_head( "http:$url" ) ) !== 200 ) {

The correct way would be to

$_remoteUrl = wp_remote_head( "http:$url" );
if ( wp_remote_retrieve_response_code( $_remoteUrl ) !== 200 ) {

https://wordpress.org/plugins/use-google-libraries/