Use PHP 7.3 compatible html dom parser

[NoodlesNZ]

Motivation

Which issue does this fix? Fixes #79

Checklist

• [x] Code follows coding conventions held in this repo
• [ ] Automated tests have been added
• [x] Tests are passing
• [ ] Docs have been updated (if applicable)
• [ ] Temporary settings (e.g. variables used during development and testing) have been reverted to defaults

How to test

Use DeliveryClient::getItem() on a PHP 7.3 system. It should not throw tons of warnings.

The sunra package for php-simple-html-dom-parser is not being actively maintained. This PR changes it to another package which is maintained. It still uses the same underlying code from Sourceforge, but a later version which has fixes for PHP 7.3.

[NoodlesNZ]

Thanks @Simply007, I added PHP 7.2 and 7.3 to the Travis config just for completeness.