fix const_get usage for rubies < 2.0.0
fix const_get usage for rubies < 2.0.0

TiagoCardoso1983/celluloid,celluloid/celluloid,jasonm23/celluloid,kenichi/celluloid,dilumn/celluloid,marshall-lee/celluloid,davydovanton/celluloid,olleolleolle/celluloid,seuros/celluloid