fix breakage introduced in apt 2.5.0

here's a proposed fix for issue #84
After this change, the resource becomes:

```ruby
# Run apt-get update to create the stamp file
execute 'apt-get-update' do
  command 'apt-get update'
  ignore_failure true
  only_if { apt_installed? }
  not_if { ::File.exists?('/var/lib/apt/periodic/update-success-stamp') }
end
```

If somebody notifies `"execute[apt-get-update]"` and the *update-success-stamp* file exists, should not be updated? Perhaps we need a third *apt-get update* resource, like previously, but with different names.