* Bump required PHP version to 7.2
* Update rest dependencies to meet php 7.2
Latest phpunit 9 requires PHP 7.3, so I've set phpunit 8 as a fallback.
* Fix TestCase inheritance
* Add phpunit cache file to gitignore
* Put license @phpdoc into separate mustache
* Bump readme PHP version to 7.2
* Bump @phpdoc PHP version to 7.2
* Update Zend Diactoros with suggested package
* Refresh samples
* Remove broken tests
These tests will be fixed in next PR which moves Mock feature to
external repo.
* Point root Travis CI environment to PHP 7.3
