Require 3.7 phpunit branch and improve phpunit.xml

As of 4.0 static testing has been [deprecated](https://github.com/sebastianbergmann/phpunit/wiki/Release-Announcement-for-PHPUnit-4.0.0#backwards-compatibility-issues)
Merge pull request #36 from jrschumacher/enhance-tests

Require 3.7 phpunit branch and improve phpunit.xml