remove what had become a lie with the release of System-Command v1.03

The following trick:

    $out = Git::Repository->command( ... )->stdout;

should probably be discouraged until System::Command is fixed (if ever),
as it currently creates zombie processes.
avoid creating zombie processes with a broken System::Command
typo fix
require the latest System-Command, as it properly works with Plack et al.
Changes for version 1.307