Require_once errors when not using composer

[blobaugh]

Currently if the library is used standalone there will be fatal errors due to missing classes. This happened when composer support was added, and support for the standalone library was removed. It can be temporarily solve by reverting the Meetup.php file to
https://github.com/blobaugh/Meetup-API-client-for-PHP/blob/804a5e9b0b82b1697321d5e57b7e4674d3aff025/Meetup.php

However an actual solution needs to be found