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/helpers/parseHeaders.js

@@ -34,8 +34,8 @@ module.exports = function parseHeaders(headers) {
 
   utils.forEach(headers.split('\n'), function parser(line) {
     i = line.indexOf(':');
-    key = utils.trim(line.substr(0, i)).toLowerCase();
-    val = utils.trim(line.substr(i + 1));
+    key = line.substr(0, i).trim().toLowerCase();
+    val = line.substr(i + 1).trim();
 
     if (key) {
       if (parsed[key] && ignoreDuplicateOf.indexOf(key) >= 0) {