fix(changeStreams): fixing node4 issue with util.inherits (#1587)

require('util').inherits(ctor, super) blows away the original prototype
of ctor. If we wish to use it, we need to move the inherits call to
directly after the constructor creation. In the future, we should
switch to using classes.

lib/change_stream.js

@@ -71,6 +71,8 @@ var ChangeStream = function(collection, pipeline, options) {
   });
 };
 
+inherits(ChangeStream, EventEmitter);
+
 // Create a new change stream cursor based on self's configuration
 var createChangeStreamCursor = function(self) {
   if (self.resumeToken) {
@@ -354,6 +356,4 @@ var processNewChange = function(self, err, change, callback) {
  * @param {(object|null)} result The result object if the command was executed successfully.
  */
 
-inherits(ChangeStream, EventEmitter);
-
 module.exports = ChangeStream;