Bundles vulnerable copy of Expat — please upgrade to Expat 2.2.7

Hi!

This repository [bundles an outdated vulnerable copy of Expat](https://github.com/pocoproject/poco/blob/master/XML/src/xmlparse.cpp). Please update your copy to version 2.2.7 with the latest security fixes. A change log with details is available at https://github.com/libexpat/libexpat/blob/master/expat/Changes . Thank you!

Best

&nbsp;
Sebastian
