get rid of config

I noticed after https://github.com/jordanmccullough/HydeSlides/pull/15 I was still getting some notices:

```
$ jekyll serve --watch
Configuration file: /Users/yossef/dev/scratch/HydeSlides/_config.yml
       Deprecation: Auto-regeneration can no longer be set from your configuration file(s). Use the --watch/-w command-line option instead.
       Deprecation: The 'server' configuration option is no longer accepted. Use the 'jekyll serve' subcommand to serve your site with WEBrick.
```

This is because of the `server` and `auto` keys in the config file. Now I just see

```
$ jekyll server --watch
Configuration file: none
```