Mark phpunit-mock-objects-3.2.0 as conflict

PHPUnit released phpunit-mock-objects-3.2.0 which breaks historic
PHPUnit versions (5.2.12, 5.1.7 and 5.0.10). These PHPUnit versions
require that dependency with no upper bound. Tests are failing with:

> Call to undefined method PHPMockTest::registerMockObject()

PHPUnit fixed the issue with a phpunit-mock-objects-3.2.1 release.
However said versions will not receive a patch, as PHPUnit considers
them already as unsupported. This commit marks the breaking
phpunit-mock-objects-3.2.0 dependency as a conflict.

See also: sebastianbergmann/phpunit-mock-objects#308

Author: malkusch

composer.json

@@ -23,5 +23,8 @@
     },
     "archive": {
         "exclude": ["/tests"]
+    },
+    "conflict": {
+        "phpunit/phpunit-mock-objects": "3.2.0"
     }
 }