Update supported node version to >= 4.0

[jwgoh]

https://github.com/visionmedia/superagent/blob/master/History.md#300-2016-11-19

[caicai0]

I used it for loading.
var loadingUrl = [];
function load(url){
loadingUrl.push(url);
superagent.get(url).end(function(err,res){
loadingUrl.remove(url);
//other code
});
}

when I used this load() function too many times for diffrent urls.I find that there are away some urls in loadingUrl.I do not know why

[kornelski]

@caicai0 Try using constructs like Promise.all() or use async/await.

However, please do not post in these comments, as they are about a pull request, which is totally unrelated to your problem.