Merge pull request #157 from crafatar/greenkeeper-canvas-1.3.9

canvas@1.3.9 breaks build 🚨

package.json

@@ -34,7 +34,7 @@
     "node": "4.2.4"
   },
   "dependencies": {
-    "canvas": "^1.3.7",
+    "canvas": "^1.3.9",
     "crc": "~3.4.0",
     "ejs": "^2.3.4",
     "lwip": "~0.0.8",