Update Travis configuration for Java 8

This commit is cherry picked from commits
 * a474a68
 * a5aa614
in https://github.com/hbz/metafacture-core/

Author: cboehme

.travis.yml

@@ -1,6 +1,6 @@
 language: java
 jdk:
-  - openjdk6
+  - oraclejdk8
 
 env:
   global:

src/test/java/org/culturegraph/mf/stream/pipe/sort/AbstractTripleSortTest.java

@@ -74,7 +74,7 @@ public void issue192ShouldUnregisterFromTheJVMToNotCauseMemoryLeak() throws Inte
 		System.gc();
 
 		// Wait a tiny bit since GC Thread is not immediately done in Java 8
-		Thread.sleep(1);
+		Thread.sleep(10);
 
 		assertTrue(weakRef.isEnqueued());
 	}