Remove redundant version number in comment

When we move to modules we can move the version constant up to the top of
the file. For now, it's one more redundant thing to update and get out of
sync with the actual version in the code.

mixpanel.js

@@ -1,5 +1,5 @@
 /*
- * Mixpanel JS Library v2.6.1
+ * Mixpanel JS Library
  *
  * Copyright 2012, Mixpanel, Inc. All Rights Reserved
  * http://mixpanel.com/