chore(package): update autoprefixer to version 6.7.7

Closes #1931

https://greenkeeper.io/

package.json

@@ -216,7 +216,7 @@
     "webpack-isomorphic-tools": "2.6.6"
   },
   "devDependencies": {
-    "autoprefixer": "6.7.5",
+    "autoprefixer": "6.7.7",
     "babel-core": "6.23.1",
     "babel-eslint": "7.1.0",
     "babel-gettext-extractor": "git+https://github.com/muffinresearch/babel-gettext-extractor.git#ecd85dea4553247a3cb579da817df29f381df5b9",