Update Webpack + deps, remove now unnecessary polyfills (#2410)

* Update deps

 * handles webpack 1 -> 4 migration

* remove promise helpers from dev files

assume `Promise` is available, or polyfilled by
the consumer

* Remove isArray util. `isArray` has good coverage, even
   in IE9. So lets remove the custom polyfill.

 https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/isArray

also resolves a few lint issues

* Remove trim util

String.protoype.trim has good coverage (including IE9)

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/Trim

Also, the http adapter already uses the native method.

test/manual/basic.html

@@ -7,7 +7,6 @@
 
 An alert should be shown with the <code>{"name":"axios"}</code>
 
-<script src="promise.js"></script>
 <script src="../../dist/axios.js"></script>
 <script>
     axios.get('./fixture.json').then(function(response) {
@@ -18,4 +17,4 @@ An alert should be shown with the <code>{"name":"axios"}</code>
 </script>
 
 </body>
-</html>
+</html>