add XSD to PHPUnit configuration

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | --
| License       | MIT
| Doc PR        | --

The syntax check functionality has been removed in PHPUnit 3.6 already. But there's no Composer constraint for PHPUnit, so you can never know which version will actually be used to run tests. Let me know what you think.