So setup InfiniteWP on one of your domains in a sub folder and then add this plugin to all of the WordPress sites. Copy over the data it gives you when activated to setup that site with the software. Now you are all set to update any plugin, theme or WordPress version in under 5 minutes.

I get up, open InfiniteWP, give it a minute to reload all of the site data, select what I want upgraded and let it do its thing. It's all finished before my 3rd sip of coffee.

I like that program because it's hosted on MY server, and not on some site that I have to go to and log in and pay a monthly fee to that may or may not be there tomorrow. I have set it up with all of my WordPress sites and some of my client's blogs (NOT just blogs, but any site I use it as a CMS on also), and I have one control panel to manage them with. It's also free.