Remove Query Strings from Static Resources

https://wordpress.org/plugins/query-strings-remover/

Query Strings Remover removes query strings from your static resources like CSS and JavaScript files. It will improve your cache performance and overall score in Google PageSpeed, YSlow, Pingdom and GTmetrix. Just install and forget everything, as no configuration needed.

After installing Query Strings Remover sure empty you cached resources in order to see the magic of this plugin.