Support installing paratest 3.x with php 7.1
Support installing paratest 3.x with php 7.1

PHP 7.1 will continue to receive security support until December 2019,
and some users of paratest would have apps supporting that
(including myself)

- Paratest seems to work with *both* phpunit 7 and phpunit 8
  (e.g. the xml files and the format this is parsing are unchanged)

  I'm assuming there aren't any 2.x backport plans.
- It doesn't seem as if paratest has started using php 7.2's new features

Fixes #389

brianium/paratest,brianium/paratest