Set Slim dependency to 2.3.2 because of issues with routes

It seems that after upgrading to Slim 2.3.3 the routes no longer
work. That is because Slim sets the 'PATH_INFO' variable to also
include the Apache defined alias together with the resource
name.
Allow only XHR requests to sevrer resources

This change is actually commented out since (of course) it would
provide pretty no sensible resource access control.