use openjdk7 in travis to ensure it works with jdk7

wing328 · wing328 · commit bedd4c880e1e · 2018-01-07T11:04:48.000+08:00
diff --git a/.travis.yml b/.travis.yml
@@ -1,7 +1,7 @@
 sudo: required
 language: java
 jdk:
-  - oraclejdk8
+  - openjdk7
 
 cache:
   directories:
