second try to fix random memcache failure on travis

tests/unit/framework/caching/CacheTestCase.php

@@ -145,14 +145,9 @@ abstract class CacheTestCase extends TestCase
 		$cache = $this->getCacheInstance();
 
 		$this->assertTrue($cache->set('expire_test', 'expire_test', 2));
-		sleep(1);
+		usleep(500000);
 		$this->assertEquals('expire_test', $cache->get('expire_test'));
-		// wait a bit more than 2 sec to avoid random test failure
-		if (isset($_ENV['TRAVIS']) && substr(StringHelper::basename(get_class($this)), 0, 8) == 'MemCache') {
-			sleep(3); // usleep with 2,5 seconds does not work well on travis and memcache
-		} else {
-			usleep(2500000);
-		}
+		usleep(2500000);
 		$this->assertFalse($cache->get('expire_test'));
 	}