Update canvas

Author: adroitwhiz

package.json

@@ -41,7 +41,7 @@
   },
   "devDependencies": {
     "acorn": "~0.5.0",
-    "canvas": "^2.4.1",
+    "canvas": "^2.6.0",
     "del": "^4.1.0",
     "gulp": "^3.9.1",
     "gulp-cached": "^1.1.0",