Update package.json to not include engine and npm

npm is printing warnings when installing this now in node 10.x.0. 

Given the simplicity of this module, these restrictions seem a bit unnecessary perhaps? Curious of your thoughts. It'd be great if you would consider making them more lenient at least. 

Thanks for extracting this module, very useful!
Merge pull request #5 from HenrikJoreteg/patch-1

Update package.json to not include engine and npm