chore(package): update grunt-contrib-imagemin to version 3.1.0

Closes #26

Author: greenkeeper[bot]

package.json

@@ -13,7 +13,7 @@
     "grunt-contrib-copy": "^1.0.0",
     "grunt-contrib-cssmin": "^2.2.0",
     "grunt-contrib-htmlmin": "^2.4.0",
-    "grunt-contrib-imagemin": "^2.0.1",
+    "grunt-contrib-imagemin": "^3.1.0",
     "grunt-contrib-jshint": "^1.1.0",
     "grunt-contrib-uglify": "^3.0.1",
     "grunt-contrib-watch": "^1.0.0",