Add autoprefixer dependency

Solves:

FATAL Something's wrong. Maybe you can find the solution here:
http://hexo.io/docs/troubleshooting.html
Error: Cannot find module 'autoprefixer'
    at Function.Module._resolveFilename (module.js:337:15)
    at Function.Module._load (module.js:287:25)
    at Module.require (module.js:366:17)
    at require
(/Users/kvz/code/uppy/website/node_modules/hexo/lib/hexo/index.js:213:21
)
    at
/Users/kvz/code/uppy/website/node_modules/hexo-renderer-postcss/index.js
:6:20

website/package.json

@@ -6,6 +6,7 @@
     "version": "3.1.1"
   },
   "dependencies": {
+    "autoprefixer": "^6.1.2",
     "hexo": "^3.1.1",
     "hexo-deployer-git": "0.0.4",
     "hexo-generator-archive": "^0.1.2",