fix(collection): ensure dynamic require of db is wrapped in parentheses

This is a fix for issue webpack/webpack#6191
Needed to surround the require statement with parenthesis to make its priority higher.

Author: bojidaryovchev

lib/collection.js

@@ -3001,7 +3001,7 @@ var mapReduce = function(self, map, reduce, options, callback) {
     if (result.result != null && typeof result.result === 'object') {
       var doc = result.result;
       // Return a collection from another db
-      collection = new require('./db')(
+      collection = new (require('./db'))(
         doc.db,
         self.s.db.s.topology,
         self.s.db.s.options