Deprecated jekyll command line in README.md - Revert 722f50d

[eyecatchup]

Not sure why this was merged at all, but 722f50d replaces jekyll serve by jekyll --server --auto.

Here's what jekyll has to say:

>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       Deprecation: The --server command has been replaced by the 'serve' subcommand.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'.
jekyll 2.5.1 | Error:  Whoops, we can't understand your command.
jekyll 2.5.1 | Error:  invalid option: --server
jekyll 2.5.1 | Error:  Run your command again with the --help switch to see available options.

Easy fix: revert 722f50d