Thanks for your pull request! This condition now seems quite cryptic, "not if the file does not exist"

"only_if" should be available to all resources would this read better as:

```ruby
only_if do
  File.exists?("#{new_resource.project_path}/composer.json")
end
```

This reads as "only if this fil exists" and is more explicit?

Did you experience problems with the condition or the resource functionality?