fix for webpack 4

package.json

@@ -10,7 +10,7 @@
     "node": ">=4"
   },
   "peerDependencies": {
-    "webpack": "2 || 3"
+    "webpack": "4"
   },
   "dependencies": {
     "fs-extra": "^5.0.0",

src/webpack-mjml-store.js

@@ -1,6 +1,6 @@
 'use strict';
 
-const mjml2html = require('mjml').default;
+const mjml2html = require('mjml');
 const glob = require('glob');
 const fs = require('fs-extra');
 const _ = require('lodash');