Updating to PHPUnit 4

- Requires not running tests that mock built-in classes without invoking
  the class constructor.
- We will need to update the tests once a new version of PHPUnit has been
  tag that addresses the issue.

See: https://github.com/php/php-src/commit/3586d14b61fbf3932650899d99a09e25784cf587
Merge pull request #308 from aws/phpunit_upgrade

Updating to PHPUnit 4