Bump version to 0.12.0
Bump version to 0.12.0

tarcieri/celluloid,marshall-lee/celluloid,mehta/celluloid,jstoja/celluloid,seuros/celluloid,sideci-sample/sideci-sample-celluloid,kenichi/celluloid,TiagoCardoso1983/celluloid,davydovanton/celluloid,celluloid/celluloid,dilumn/celluloid,jasonm23/celluloid,olleolleolle/celluloid