Downgrade hibernate-core to 4.1.9.Final

http://stackoverflow.com/questions/16058583/nullpointerexception-when-persisting-entity

http://stackoverflow.com/a/16085943/1433665

OPEN - task 57: Existing Category Update gives NullPointerException 
http://github.com/DevOpsDistilled/OpERP/issues/issue/57
Merge branch 'iss57-update-category' into develop