[Plugin: Google Analytics for WordPress] Specify an expiration

I am trying to improve loading times for my site, the page speed report for GTMetrix indicates that I need to “Specify an expiration at least one week in the future for the following resources:

http://www.google-analytics.com/ga.js (12 hours)”

any suggestions on how I can go about this?

http://wordpress.org/extend/plugins/google-analytics-for-wordpress/