chore(package): update autoprefixer to version 9.7.6

Closes #56

Author: greenkeeper[bot]

package.json

@@ -44,7 +44,7 @@
     "@vue/server-test-utils": "^1.0.0-beta.26",
     "@vue/test-utils": "^1.0.0-beta.18",
     "@vuedoc/md": "^1.5.0",
-    "autoprefixer": "^8.5.2",
+    "autoprefixer": "^9.7.6",
     "babel-core": "^6.22.1",
     "babel-eslint": "^8.2.6",
     "babel-helper-vue-jsx-merge-props": "^2.0.3",