Upgrade to servlet api 3.1.0

In Vaadin 7.7.0 import versions of package javax.servet was fixed around range [2.5, 3.0.1]. Before that there is no upper bound of version, so Vaadin works perfectly with servlet api 3.1.0.
So, last version of Vaadin couldn't be used with Pax Web, a part of Karaf Container, because Pax Web exports only version 3.1.0 of servlet api packages.
Maybe it is a good reason to force dependency upgrade of servlet api :)
P.S.: javax.servlet-api 3.1.0 available from central maven repo from 26-Apr-2013

Hey Alexander!

I suggest that you file this in our ticket system to make sure it gets the attention of the dev team: https://dev.vaadin.com/ 

Or, even better, consider contributing a patch: https://github.com/vaadin/vaadin/blob/master/CONTRIBUTING.md

Thanks!