Default to the 'help' command if no subcommand specified
Default to the 'help' command if no subcommand specified

This fixes an ugly error if the user runs 'vagrant landrush'
with no further arguments.

vagrant-landrush/landrush,vagrant-landrush/landrush,strzibny/landrush,njam/landrush,mheiges/landrush,phinze/landrush,sneal/landrush,LalatenduMohanty/landrush