update travis file

travis not support openjdk6 and oraclejdk7, revise it.

Author: aaronshan

.travis.yml

@@ -1,9 +1,8 @@
 language: java
 
 jdk:
-  - oraclejdk7
+  - oraclejdk8
   - openjdk7
-  - openjdk6
 
 before_cache:
   # Make the cache stable between builds by removing build output
@@ -18,9 +17,7 @@ install:
   - mvn install:install-file -DgroupId=javax.jdo -DartifactId=jdo2-api -Dversion=2.3-ec -Dpackaging=jar -Dfile=$HOME/jdo2-api-2.3-ec.jar
 
 script:
-  - jdk_switcher use openjdk6
-  - mvn clean package
   - jdk_switcher use openjdk7
   - mvn clean package
-  - jdk_switcher use oraclejdk7
+  - jdk_switcher use oraclejdk8
   - mvn clean package