installation: cerberus package upgrade

* Upgrades Cerberus package in order to support some new features.

Signed-off-by: Konstantinos Kostis <konstantinos.kostis@cern.ch>
restful: fix for testing accesstoken

* Renames class RestValidator to RESTValidator.

Signed-off-by: Konstantinos Kostis <konstantinos.kostis@cern.ch>
tags: fix for editor in search results

* Fixes broken javascript editor.  (addresses #1792)

Signed-off-by: Eirini Psallida <eirini.psallida@cern.ch>