Implemented EZP-20816: Added new index on ezcontentobject_attribute

Reflected changes on version update scripts
Pushing the enhancement to 5.3
Set version in postgre update script

Ref: https://jira.ez.no/browse/EZP-20816
Merge pull request #772 from ezsystems/ezp20816_index_contentclassattribute_id

Implemented EZP-20816: Added new index on ezcontentobject_attribute