Feature/update dependencies

[jantimon]

Adjust travis for: #11
Remove node < 6 support

[ninetails]

Now much better with class :D 👍

One thing that I want to explain due to my last submit was about named chunks, I needed to do that because bundle.js was default only on webpack@1.x.x, since 2.2.0 I saw here on tests they've changed to main.js. I don't know any alternatives for tests to work on all those versions with same test file but I've liked using webpack's default.

About pull request, I'd like to suggest to update test file also to node ES6. Things that I've tested and ran with no errors was replace var to const and functions to arrow functions.

And on source, extract functions (resourceHintWebpackPluginAlterAssetTags and addResourceHintTags) from class (and getting rid of bind, both of them don't use this), but I'm not 100% sure about this, tried as on this gist, just extracting them and removing bind but please confirm this too.

[jantimon]

Yes I really like your solution - I'll merge the current version for now so you can create a new pull request with your improvements