Added CanvasPattern global flag for jshint

The core.helpers file was failing linting checks as the global CanvasPattern was not defined. Added the `/* global CanvasGradient */` statement to make linting pass. Updates chartjs/Chart.js#1323

src/core/core.helpers.js

@@ -944,6 +944,7 @@ module.exports = function(Chart) {
 		}
 	};
 	helpers.getHoverColor = function(color) {
+		/* global CanvasGradient */
 		return (color instanceof CanvasPattern) ?
 			color :
 			helpers.color(color).saturate(0.5).darken(0.1).rgbString();