Use eslint for everything

eslint is much more powerful and flexible than jshint, and we don't want
to promote writing new projects with pre-ES6 code.

As a side effect, this also avoids downloading PhantomJS (see
https://github.com/jshint/jshint/issues/3318), thereby cutting down
node_modules/ from 470 MB to 210 MB.

webpack.config.js

@@ -119,12 +119,6 @@ module.exports = {
     },
     module: {
         rules: [
-            {
-                enforce: 'pre',
-                exclude: /node_modules/,
-                loader: 'jshint-loader',
-                test: /\.js$/
-            },
             {
                 enforce: 'pre',
                 exclude: /node_modules/,
@@ -134,7 +128,7 @@ module.exports = {
             {
                 enforce: 'pre',
                 exclude: /node_modules/,
-                loader: 'jshint-loader?esversion=6',
+                loader: 'eslint-loader',
                 test: /\.es6$/
             },
             {