Update lib/Doctrine/ODM/PHPCR/Mapping/ClassMetadata.php

Removed exception, because we can't know the versionable attribute if an inherited field is mapped.
see https://github.com/doctrine/phpcr-odm/blob/master/lib/Doctrine/ODM/PHPCR/Mapping/ClassMetadataFactory.php#L118
First we load inherited fields, after that we load classmetadata. So if i have an inherited versionName and versionCreated attribute
this will fail, because the metadata of the current class isnt loaded yet.