add empty namespace declaration for gulp-uglify to allow es6 imports

(see /Microsoft/TypeScript/issues/5073)

gulp-uglify/gulp-uglify.d.ts

@@ -170,6 +170,6 @@ declare module "gulp-uglify" {
          */
         comments_before: string[];
     }
-
+    namespace GulpUglify {}
     export = GulpUglify;
-}
+}