Upgrade to Grails 2.2.0

This updates the sample application to Grails 2.2.0. The previous version was causing an IllegalStateException:

ERROR [/grailstwitter].[gsp]  - Servlet.service() for servlet [gsp] in context with path [/grailstwitter] threw exception Message: No thread-bound request found: Are you referring to request attributes outside of an actual web request, or processing a request outside of the originally receiving thread? If you are actually operating within a web request and still receive this message, your code is probably running outside of DispatcherServlet/DispatcherPortlet: In this case, use RequestContextListener or RequestContextFilter to expose the current request