Use es6-promise package to avoid webpack warnings

See https://github.com/stefanpenner/es6-promise/issues/100

```
[WDS] Warnings while compiling.
./vendor/assets/javascripts/es6-promise.auto.js
Module not found: Error: Can't resolve 'vertx' in
'/Users/eric/Documents/gitlab/gitlab-development-kit/gitlab/vendor/assets/javascripts'
 @ ./vendor/assets/javascripts/es6-promise.auto.js 140:16-26
 @ ./app/assets/javascripts/application.js
 @ multi (webpack)-dev-server/client?http://localhost:3808
./application.js
```

app/assets/javascripts/application.js

@@ -56,8 +56,7 @@ requireAll(require.context('./u2f',        false, /^\.\/.*\.(js|es6)$/));
 requireAll(require.context('./droplab',    false, /^\.\/.*\.(js|es6)$/));
 requireAll(require.context('.',            false, /^\.\/(?!application\.js).*\.(js|es6)$/));
 require('vendor/fuzzaldrin-plus');
-window.ES6Promise = require('vendor/es6-promise.auto');
-window.ES6Promise.polyfill();
+require('es6-promise').polyfill();
 
 (function () {
   document.addEventListener('beforeunload', function () {

package.json

@@ -19,6 +19,7 @@
     "compression-webpack-plugin": "^0.3.2",
     "d3": "3.5.11",
     "dropzone": "4.2.0",
+    "es6-promise": "^4.0.5",
     "imports-loader": "^0.6.5",
     "jquery": "2.2.1",
     "jquery-ui": "github:jquery/jquery-ui#1.11.4",