Updated the plugin to use Spring Web Flow version 2.2.0

The current Spring Web Flow version 2.0.8 that is used by this plugin contains a critical bug that prevents this plugin being used/deployed in a clustered environment.

More information on the problem can be found here:
https://jira.springsource.org/browse/SWF-1213

And this it the commit that fixed it:
https://github.com/spring-projects/spring-webflow/commit/76c9dbba8060387541369dab8c32c187a1668ae9