added unique_together constraint to persistantconnections
added unique_together constraint to persistantconnections

unicefuganda/edtrac,peterayeni/rapidsms,lsgunth/rapidsms,catalpainternational/rapidsms,lsgunth/rapidsms,ken-muturi/rapidsms,lsgunth/rapidsms,caktus/rapidsms,peterayeni/rapidsms,peterayeni/rapidsms,eHealthAfrica/rapidsms,caktus/rapidsms,peterayeni/rapidsms,caktus/rapidsms,catalpainternational/rapidsms,dimagi/rapidsms,catalpainternational/rapidsms,eHealthAfrica/rapidsms,lsgunth/rapidsms,ehealthafrica-ci/rapidsms,ken-muturi/rapidsms,dimagi/rapidsms,ken-muturi/rapidsms,unicefuganda/edtrac,eHealthAfrica/rapidsms,ehealthafrica-ci/rapidsms,catalpainternational/rapidsms,ehealthafrica-ci/rapidsms,unicefuganda/edtrac