remove jdk7 because travis do not support it anymore, travis-ci/travis-ci#7884 (comment)

jemygraw · jemygraw · commit ac95e136e142 · 2018-01-03T13:39:45.000+08:00
diff --git a/.travis.yml b/.travis.yml
@@ -4,7 +4,6 @@ sudo: false
 language: java
 
 jdk:
-  - oraclejdk7
   - oraclejdk8
 
 before_script:
