[Tests] Use phpunit-mock-objects version 2.3.0 for HHVM compat

"Version 2.3.1 uses a different reflection methodology which is currently broken in HHVM
(see facebook/hhvm#5170).  Downgrading will allow our tests to pass while this
issue is resolved."
Ref: thephpleague/commonmark@2a6a97c

composer.json

@@ -38,6 +38,7 @@
         "ezsystems/ezpublish-legacy": "*",
         "mikey179/vfsStream": "1.1.0",
         "phpunit/phpunit": "~4.7.0",
+        "phpunit/phpunit-mock-objects": "2.3.0",
         "matthiasnoback/symfony-dependency-injection-test": "0.*",
         "mockery/mockery": "~0.9.4",
         "symfony/assetic-bundle": "~2.3"