Redo the change to uglify-es, which got lost in a merge.

When I merged devel into meteor/meteor#8698 and bumped the version, I
mistakenly overwrote the (quite important) '-es' extension on the new
UglifyJS package.  This reinstates that change, originally implemented
by 9d81f0e6c5.

packages/minifier-js/minifier.js

@@ -4,7 +4,7 @@ meteorJsMinify = function (source) {
   var result = {};
   var NODE_ENV = process.env.NODE_ENV || "development";
 
-  uglify = uglify || Npm.require("uglify-js");
+  uglify = uglify || Npm.require("uglify-es");
 
   try {
     var uglifyResult = uglify.minify(source, {

packages/minifier-js/package.js

@@ -4,7 +4,7 @@ Package.describe({
 });
 
 Npm.depends({
-  "uglify-js": "3.0.28"
+  "uglify-es": "3.0.28"
 });
 
 Package.onUse(function (api) {