Updated Rakefile :preview command to new Jekyll standards

jekyll --server --auto is deprecated and has been replaced with jekyll serve(r) -w(atch)

When running the old version, jekyll messages:
```
  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  Deprecation: The --server command has been replaced by the 'serve' subcommand.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'.
```
This commit should fix that by replacing the command with the current version