Convert require statements to imports

[bschlenk]

Converts

define(function(require) {
    var a = require('a');
    var b = require('b');

    var stuff = {};
    return stuff;
});

into

import a from 'a';
import b from 'b';

var stuff = {};
export default stuff;

[bschlenk]

Since this is a tool for moving code from es5 to es6, I think it is safe to assume that it will be used with node versions supporting es6 features. The travis builds for node 4 can probably safely be turned off without affecting consumers.