Drop remnants of HHVM from tests

It was removed from Travis in c479cdd42f

tests/Provider/Time/SystemTimeProviderTest.php

@@ -23,7 +23,6 @@ class SystemTimeProviderTest extends TestCase
      */
     public function testCurrentTimeUsesGettimeofday()
     {
-        $this->skipIfHhvm();
         $timestamp = ['sec' => 1458844556, 'usec' => 200997];
         $func = AspectMock::func('Ramsey\Uuid\Provider\Time', 'gettimeofday', $timestamp);
         $provider = new SystemTimeProvider();