Add JDK8 to Travis cache

in order to not download JDK8 installer from oracle.com
on each build.

.travis.yml

@@ -17,6 +17,7 @@ cache:
     - $HOME/.sonar
     - server/sonar-web/node
     - server/sonar-web/node_modules
+    - $HOME/jvm
 
 before_cache:
   # remove all the artifacts (JAR, ZIP) that are installed in local repo because of mvn deploy