Remove default command entirely
Remove default command entirely

Since this has been a source of confusion and many support tickets it
was decided to require `cmd:` to be explicitly set by the user, this is
the first step in that process.

Signed-off-by: Tyler Pickett <1d7399073f785129b42f375de87dfa551441ffbc@gmail.com>

baberthal/guard-rspec,mes/guard-rspec,guard/guard-rspec,consti/guard-rspec,kvokka/guard-rspec