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.

lib/core/enhanceError.js

@@ -20,7 +20,7 @@ module.exports = function enhanceError(error, config, code, request, response) {
   error.response = response;
   error.isAxiosError = true;
 
-  error.toJSON = function() {
+  error.toJSON = function toJSON() {
     return {
       // Standard
       message: this.message,