fixes error in nodejs <= 0.12
fixes error in nodejs <= 0.12

KonstantinShemyak/cytoscape.js,soomin-maeng/cytoscape.js,rlugojr/cytoscape.js,soomin-maeng/cytoscape.js,KonstantinShemyak/cytoscape.js,rlugojr/cytoscape.js