fix rails 4.2 deprecation warning in uniqueness validator
fix rails 4.2 deprecation warning in uniqueness validator

cao7113/mongoid,amw/mongoid,cao7113/mongoid,herimedia/mongoid,mongodb/mongoid,zenjoy/mongoid,uniiverse/mongoid,simi/mongoid,matbox/mongoid,joshsoftware/mongoid,johnny-miyake/mongoid,glebtv/mongoid,bencrouse/mongoid,imomoisoft/mongoid,sideci-sample/sideci-sample-mongoid,nviennot/mongoid,tricknotes/mongoid,nusco/mongoid,subhashb/mongoid,drobune/mongoid,bjori/mongoid,rpocklin/mongoid,kayakyakr/mongoid,arnkorty/mongoid,wuboy0307/mongoid,rinmu/mongoid,Azzurrio/mongoid,zBMNForks/mongoid,wuboy0307/mongoid,zBMNForks/mongoid,Welovroi/mongoid,canvas-drw/mongoid,mysociety/mongoid,rpocklin/mongoid,Sharevari-Inc/mongoid,eljojo/mongoid,mongodb/mongoid,stnguyen/mongoid,etomic/mongoid,zaithottakath/mongoid,eljojo/mongoid,narutosanjiv/mongoid,nusco/mongoid,drobune/mongoid,hxx/mongoid,zaithottakath/mongoid,infiverve/mongoid,estolfo/mongoid,mongodb/mongoid,jonhyman/mongoid,canvas-drw/mongoid,estolfo/mongoid,trmmy/mongoid,Sharevari-Inc/mongoid,infiverve/mongoid,matbox/mongoid,tricknotes/mongoid,Azzurrio/mongoid,sivagollapalli/mongoid,hxx/mongoid,kay-kim/mongoid,kcdragon/mongoid,demarque/mongoid,hashrocketeer/mongoid,sivagollapalli/mongoid,Welovroi/mongoid,hashrocketeer/mongoid,nviennot/mongoid,parov/mongoid,rafaelgaspar/mongoid,karmakaze/mongoid,simi/mongoid,asantamar/mongoid,codedogfish/mongoid,uniiverse/mongoid,imomoisoft/mongoid,demarque/mongoid,kayakyakr/mongoid,narutosanjiv/mongoid,mongoid/mongoid,rinmu/mongoid,herimedia/mongoid,subhashb/mongoid,karmakaze/mongoid,kcdragon/mongoid,mongoid/mongoid,Zhomart/mongoid,itbakery/mongoid,arnkorty/mongoid,Zhomart/mongoid,stnguyen/mongoid,asantamar/mongoid,tzar/mongoid,jbrien/mongoid,etomic/mongoid,tzar/mongoid,kay-kim/mongoid,sideci-sample/sideci-sample-mongoid,itbakery/mongoid,codedogfish/mongoid,johnny-miyake/mongoid,glebtv/mongoid,jbrien/mongoid,rafaelgaspar/mongoid,mysociety/mongoid,parov/mongoid,trmmy/mongoid,bjori/mongoid,nicolasblanco/mongoid,joshsoftware/mongoid,amw/mongoid,jonhyman/mongoid,bencrouse/mongoid,zenjoy/mongoid