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

Closes #22

Author: greenkeeper[bot]

package.json

@@ -27,7 +27,7 @@
     "grunt": "~1.0.1",
     "grunt-contrib-concat": "~1.0.1",
     "grunt-contrib-copy": "~1.0.0",
-    "grunt-contrib-imagemin": "~2.0.0",
+    "grunt-contrib-imagemin": "~3.1.0",
     "grunt-contrib-jshint": "~2.0.0",
     "grunt-contrib-sass": "~1.0.0",
     "grunt-contrib-uglify": "~4.0.0",