Add missing dependency

Author: James Kolce

package.json

@@ -18,7 +18,8 @@
     "postcss": "^5.0.21",
     "postcss-custom-media": "^5.0.1",
     "postcss-lh": "^1.1.0",
-    "postcss-media-minmax": "^2.1.2"
+    "postcss-media-minmax": "^2.1.2",
+    "postcss-input-range": "^2.0.0"
   },
   "devDependencies": {
     "tape": "^4.5.1"