chore(package): update autoprefixer to version 6.7.4

Closes #99

https://greenkeeper.io/

package.json

@@ -26,7 +26,7 @@
     "react-dom": "15.x.x"
   },
   "devDependencies": {
-    "autoprefixer": "6.7.2",
+    "autoprefixer": "6.7.4",
     "babel-core": "6.22.1",
     "babel-eslint": "7.1.1",
     "babel-loader": "6.3.0",