Fix for Confluence 4.x compatibility

The atlassian integration module is now compiled against Confluence
lib for version 4.2.8 and a small modifications have been done in
ConfluenceCasAuthenticator class, because Confluence API for custom
authenticators changed a little bit. See the problem here :
https://answers.atlassian.com/questions/75044/cas-integration-classcastexception-with-confluence-version-4-2-11