Upgrade npm dependencies

Change gulp-uglify for gulp-uglify-es which has support for ES6.

New dependencies:
* boostrap
* jquery
* popper.js (required by bootstrap)

gulpfile.js

@@ -12,7 +12,7 @@ var pug          = require('gulp-pug');
 var rename       = require('gulp-rename');
 var sass         = require('gulp-sass');
 var sourcemaps   = require('gulp-sourcemaps');
-var uglify       = require('gulp-uglify');
+var uglify       = require('gulp-uglify-es').default;
 
 var enabled = {
     uglify: argv.production,