Upgrade Servlet API to 3.1.0

[hpehl]

Running SuperDevMode for a GWTP app using GWT 2.8.0-rc1 throws a java.lang.NoSuchMethodError due to conflicting Servlet APIs: The Jetty web server bundled with 2.8.0-rc1 depends on 3.1.0 whereas GWTP still uses 2.5.0 (see also https://groups.google.com/d/msg/google-web-toolkit/S9KV1GaCyvg/N9GsmoWvAQAJ).

This PR upgrades the Servlet API to 3.1.0. Running a quick mvn clean install did not show any problems in using 3.1.0

[lukelast]

Something to keep in mind is that those of us using Google App Engine are still stuck on servlet 2.5 for feature support in production.

[christiangoudreau]

It should be alright now to upgrade though