check plugin.default and then just plugin to support ES6 modules - closes #1627

src/babel/transformation/file/plugin-manager.js

@@ -37,9 +37,10 @@ export default class PluginManager {
 
     var loc = util.resolveRelative(`babel-plugin-${name}`) || util.resolveRelative(name);
     if (loc) {
+      var plugin = require(loc);
       return {
         position: position,
-        plugin:   require(loc)
+        plugin:   plugin.default || plugin
       };
     } else {
       throw new ReferenceError(messages.get("pluginUnknown", name));