fixed showing no 'Usage'.

mruby command not shows 'Usage: [swiches] programfile' with no switches or wrong switches.

Not Fixed
$ mruby                /* no switches */
$ mruby -h             /* wrong switches */

Fixed
$ mruby
or
$ mruby -h
Usage: mruby [switches] programfile
  switches:
  -b           load and execute RiteBinary (mrb) file
  -c           check syntax only
  -e 'command' one line of script
  -v           print version number, then run in verbose mode
  --verbose    run in verbose mode
  --version    print the version
  --copyright  print the copyright