Uninstall tests incompatible with PHPUnit 6

[JDGrimes]

Missed in #19.

PHP Fatal error:  Class 'PHPUnit_Framework_Constraint' not found in /home/travis/build/WordPoints/wordpoints/vendor/jdgrimes/wpppb/src/WPPPB/Constraint/TableExists.php on line 16

PHP Stack trace:

PHP   1. {main}() /home/travis/.composer/vendor/phpunit/phpunit/phpunit:0
PHP   2. PHPUnit\TextUI\Command::main() /home/travis/.composer/vendor/phpunit/phpunit/phpunit:53
PHP   3. PHPUnit\TextUI\Command->run() /home/travis/.composer/vendor/phpunit/phpunit/src/TextUI/Command.php:148
PHP   4. PHPUnit\TextUI\TestRunner->doRun() /home/travis/.composer/vendor/phpunit/phpunit/src/TextUI/Command.php:195
PHP   5. PHPUnit\Framework\TestSuite->run() /home/travis/.composer/vendor/phpunit/phpunit/src/TextUI/TestRunner.php:537
PHP   6. PHPUnit\Framework\TestSuite->run() /home/travis/.composer/vendor/phpunit/phpunit/src/Framework/TestSuite.php:744
PHP   7. WordPoints_Uninstall_Test->run() /home/travis/.composer/vendor/phpunit/phpunit/src/Framework/TestSuite.php:744
PHP   8. PHPUnit\Framework\TestResult->run() /home/travis/.composer/vendor/phpunit/phpunit/src/Framework/TestCase.php:894
PHP   9. WordPoints_Uninstall_Test->runBare() /home/travis/.composer/vendor/phpunit/phpunit/src/Framework/TestResult.php:698
PHP  10. WordPoints_Uninstall_Test->runTest() /home/travis/.composer/vendor/phpunit/phpunit/src/Framework/TestCase.php:939
PHP  11. ReflectionMethod->invokeArgs() /home/travis/.composer/vendor/phpunit/phpunit/src/Framework/TestCase.php:1071
PHP  12. WordPoints_Uninstall_Test->test_uninstall() /home/travis/.composer/vendor/phpunit/phpunit/src/Framework/TestCase.php:1071
PHP  13. WPPPB_TestCase_Uninstall::assertTableExists() /home/travis/build/WordPoints/wordpoints/tests/phpunit/tests/uninstall.php:81
PHP  14. WPPPB_TestCase_Uninstall::isInDatabase() /home/travis/build/WordPoints/wordpoints/vendor/jdgrimes/wpppb/src/WPPPB/TestCase/Uninstall.php:287
PHP  15. spl_autoload_call() /home/travis/build/WordPoints/wordpoints/vendor/jdgrimes/wpppb/src/WPPPB/TestCase/Uninstall.php:414
PHP  16. xrstf_Composer52_ClassLoader->loadClass() /home/travis/build/WordPoints/wordpoints/vendor/jdgrimes/wpppb/src/WPPPB/TestCase/Uninstall.php:414
PHP  17. include() /home/travis/build/WordPoints/wordpoints/vendor/composer/ClassLoader52.php:186