Snockets output is dependent on file list ordering when using require_tree

`require_tree` uses `fs.readdir` and `fs.readdirSync`, but `snockets` doesn't sort these files before it does the concatenation. This is causing an issue for me because I'm using my app with Heroku. I have 2 dynos running and both are generating different static assets with different static asset md5 hashes. Depending on which dyno you hit, you might get a 404 when you request a specific asset. I believe that each dyno is doing it's `require_tree` in a different order, therefore the content and hash ends up being different.