Fix memory leak

This problem was introduced by 8189925
as determined by git bisect: https://gist.github.com/felixge/5567854

Fixes #471

Author: felixge

lib/protocol/sequences/Sequence.js

@@ -55,6 +55,13 @@ Sequence.prototype.end = function(err) {
     this._addLongStackTrace(err);
   }
 
+  // Without this we are leaking memory. This problem was introduced in
+  // 8189925374e7ce3819bbe88b64c7b15abac96b16. I suspect that the error object
+  // causes a cyclic reference that the GC does not detect properly, but I was
+  // unable to produce a standalone version of this leak. This would be a great
+  // challenge for somebody interested in difficult problems : )!
+  delete this._callSite;
+
   // try...finally for exception safety
   try {
     if (err) {