Compatibility with PHP 8.1 & 8.2 (#29)

Bumped min PHP version to PHP 5.6

Test PHP 7.1 and remove support for HHVM

Added stylesheet setter. Closes #8

Cleanup

Add PHP 7 to travis tests

Fixed #3 DOMDocument::loadXML() error for URIs with all components

CRLF to LF

Fixed composer.json keyword (again?)