update travis to trusty (#799)

Switching to test only against OpenJDK as support in travis has
shifted.

Fixes: #793

.travis.yml

@@ -1,14 +1,13 @@
 sudo: false
-dist: precise
+dist: trusty
 language: java
 addons:
   # https://github.com/travis-ci/travis-ci/issues/5227#issuecomment-165131913
   hosts:
     - fake-hostname-to-work-around-travis-bug
   hostname: fake-hostname-to-work-around-travis-bug
 jdk:
-  - oraclejdk8
-  - oraclejdk7
+  - openjdk8
   - openjdk7
 install: ./gradlew assemble
 script: ./gradlew build install