Fixing help command in deprecation warning

There appears to be a long-standing inconsistency in the deprecation warning regarding how to use the help command:

```
Deprecation: Jekyll now uses subcommands instead of just switches. Run `jekyll help' to find out more.
```

Leading to much hilarity:

```
$ jekyll help
Invalid command. Use --help for more information
```
Correct me if I'm wrong, but it appears that the intention was to retain the `--help` and `--version` flags, and to assume that anything else is a subcommand:

```ruby
if args.size > 0 && args.first =~ /^--/ && !%w[--help --version].include?(args.first)
    ...
end
```