fix(package): update imagemin-pngquant to version 8.0.0

Closes #37

Author: greenkeeper[bot]

package.json

@@ -41,7 +41,7 @@
     "gulp-refresh": "^1.1.0",
     "imagemin": "^5.2.1",
     "imagemin-mozjpeg": "^7.0.0",
-    "imagemin-pngquant": "^5.0.0",
+    "imagemin-pngquant": "^8.0.0",
     "inflection": "^1.12.0",
     "less": "^2.7.1",
     "less-plugin-clean-css": "^1.5.1",