Relax LiveVersionMapTests.testRamBytesUsed.

With Java9's new restrictions we cannot compute ram usage as accurately as
before. See https://issues.apache.org/jira/browse/LUCENE-7595.

core/src/test/java/org/elasticsearch/index/engine/LiveVersionMapTests.java

@@ -36,8 +36,8 @@ public class LiveVersionMapTests extends ESTestCase {
         }
         long actualRamBytesUsed = RamUsageTester.sizeOf(map);
         long estimatedRamBytesUsed = map.ramBytesUsed();
-        // less than 25% off
-        assertEquals(actualRamBytesUsed, estimatedRamBytesUsed, actualRamBytesUsed / 4);
+        // less than 50% off
+        assertEquals(actualRamBytesUsed, estimatedRamBytesUsed, actualRamBytesUsed / 2);
 
         // now refresh
         map.beforeRefresh();