Running the PHPUnit tests under HHVM causes a stacktrace:
```
ValidateSchemaTest::testValidEvent
JsonSchemaException: Invalid node: expecting "string", got "boolean". Path: "Root node -> clientValidated"
extensions/EventLogging/includes/JsonSchema.php:425
extensions/EventLogging/includes/JsonSchema.php:458
extensions/EventLogging/includes/JsonSchema.php:432
extensions/EventLogging/EventLogging.php:146
extensions/EventLogging/tests/ValidateSchemaTest.php:118
tests/phpunit/MediaWikiTestCase.php:141
```
Dummy change used: https://gerrit.wikimedia.org/r/#/c/70126/
Failure: https://integration.wikimedia.org/ci/job/mwext-EventLogging-testextension-hhvm/1/testReport/
It passes under Zend: https://integration.wikimedia.org/ci/job/mwext-EventLogging-testextension-zend/6/testReport/