Replace "ES2015/ES6 ES2016/ES7" with "ES2015/ES6+"

[UltCombo]

This is shorter, easier to read and understand, and also future-proof — that is, when we start using proposals for ES2017 and beyond, or if some of the proposals we are making use of don't make it to ES2016, then we don't have to edit the text.

I believe the header is the only place we have to fix:

@jaydson @robsongajunior WDYT?