fix darwin spec

At master b89bbf8e79f060be3e016a9620b9fbf7d0dc0440, I've got following error when I execute `bundle exec rake spec`:

```log
Failures:

  1) Port "80" command should eq "netstat -tunl | grep -- :80\\ "
     Failure/Error: its(:command) { should eq 'netstat -tunl | grep -- :80\\ ' }
       
       
       expected: "netstat -tunl | grep -- :80\\ "
            got: "lsof -nP -iTCP -sTCP:LISTEN | grep -- :80\\ "
       
       (compared using ==)
     # ./spec/darwin/port_spec.rb:7:in `block (2 levels) in <top (required)>'

  2) Service "sshd" command should eq "service sshd status"
     Failure/Error: its(:command) { should eq "service sshd status" }
       
       
       expected: "service sshd status"
            got: "launchctl list | grep sshd | grep -E '^[0-9]+'"
       
       (compared using ==)
     # ./spec/darwin/service_spec.rb:23:in `block (2 levels) in <top (required)>'
```