drop jdk7. use openjdk

travis-ci/travis-ci#10290 (comment)

> oraclejdk8 is not preinstalled on Xenial anymore, and cannot be retrieved from Oracle itself anymore. We'd suggest using either openjdk, or the currently supported Oracle JDK.

Author: xuwei-k

Diff for: .travis.yml

@@ -3,8 +3,7 @@ language: scala
 sudo: false
 
 jdk:
-  - openjdk7
-  - oraclejdk8
+  - openjdk8
 
 script:
   - git config core.whitespace tab-in-indent,trailing-space,space-before-tab,cr-at-eol