Merge pull request #1857 from mozilla/greenkeeper/autoprefixer-6.7.4

Update autoprefixer to the latest version 🚀

Author: tofumatt

package.json

@@ -213,7 +213,7 @@
     "webpack-isomorphic-tools": "2.6.6"
   },
   "devDependencies": {
-    "autoprefixer": "6.7.3",
+    "autoprefixer": "6.7.4",
     "babel-core": "6.23.1",
     "babel-eslint": "7.1.0",
     "babel-gettext-extractor": "git+https://github.com/muffinresearch/babel-gettext-extractor.git#ecd85dea4553247a3cb579da817df29f381df5b9",