Use babel env preset, simplify testing pipeline

With the new Node LTS as our baseline we don't really have to
do extra transpilation. ava still does its own but it does so
inconsistently ( avajs/ava#1319 ),
so we've switched back from import to require(). Indeed ava
might get an option to turn off transpilation altogether
( avajs/ava#1556 ), which we
may end up using if it doesn't come with bad tradeoffs.

.gitignore

@@ -51,9 +51,6 @@ config/development.json5
 static/js
 static/css/editor
 
-# Generated
-*-es5.js*
-
 # Built via jsdoc
 static/devdocs/frontend
 static/devdocs/backend

Gruntfile.js

@@ -41,24 +41,10 @@ module.exports = function(grunt) {
       }
     },
     babel: {
-      tests: {
-        options: {
-          sourceMap: true,
-          plugins: ['transform-runtime'],
-          presets: ['es2015', 'stage-2']
-        },
-        files: {
-          'tests/helpers/model-helpers-es5.js': 'tests/helpers/model-helpers.js',
-          'tests/helpers/integration-helpers-es5.js': 'tests/helpers/integration-helpers.js',
-          'tests/helpers/content-helpers-es5.js': 'tests/helpers/content-helpers.js',
-          'tests/helpers/test-helpers-es5.js': 'tests/helpers/test-helpers.js',
-          'tests/fixtures/db-fixture-es5.js': 'tests/fixtures/db-fixture.js'
-        }
-      },
       mainJS: {
         options: {
           sourceMap: true,
-          presets: ['es2015']
+          presets: ['env']
         },
         files: {
           'static/js/libreviews.js': 'frontend/libreviews.js',