update jekyll command

I tried the old command and got deprecation warnings

```
$ jekyll --server --auto
       Deprecation: Jekyll now uses subcommands instead of just switches. Run `jekyll help' to find out more.
       Deprecation: The --server command has been replaced by the 'serve' subcommand.
       Deprecation: The switch '--auto' has been replaced with '--watch'.
invalid option: --server
```

The new hotness is

```
jekyll serve --watch
```