Update travis yml to pull in java8 a different way (#8)

Also only build on master branch and pull requests

.travis.yml

@@ -1,8 +1,13 @@
 language: node_js
 sudo: required
 jdk: oraclejdk8
-before_script:
-  - sudo apt-get update && sudo apt-get install oracle-java8-installer
+addons:
+  apt:
+    packages:
+      - oracle-java8-set-default
+branches:
+  only:
+    - master
 node_js:
   - '6'
   - '6.9'