fix(package): update imagemin-pngquant to version 9.0.0

Closes #18

Author: greenkeeper[bot]

package.json

@@ -26,7 +26,7 @@
     "imagemin-gifsicle": "6.0.1",
     "imagemin-jimp": "github:jaeh/imagemin-jimp",
     "imagemin-mozjpeg": "8.0.0",
-    "imagemin-pngquant": "7.0.0",
+    "imagemin-pngquant": "9.0.0",
     "imagemin-svgo": "7.0.0",
     "jimp": "0.6.4",
     "node-zopfli-es": "1.0.3",