Promote cmp usage as it provides results with octal mode

[alexpop]

The default rspec eq matcher outputs the expected mode as decimal:

its('mode') { should eq 0644 }

=>

 File /etc/ssh/ssh_config
     ✔  should be mode 420

where the inspec cmp matcher is outputting it as octal:

its('mode') { should cmp '0644' }

=>

  File /etc/ssh/ssh_config
     ✔  mode should cmp "0644"

[chris-rock]

Thanks @alexpop for this promotion :-)