fix(conflict): fix the conflict between maven compiler level 1.8 and travis openjdk7

Author: vongosling

.travis.yml

@@ -1,11 +1,6 @@
 dist: trusty
 
 language: java
-
-addons:
-  apt:
-    packages:
-      - openjdk-6-jdk
 
 install:
   - echo "Downloading Maven 3.0";
@@ -20,11 +15,10 @@ jdk:
 - oraclejdk8
 - oraclejdk9
 - oraclejdk11
-- openjdk7
 - openjdk8
 
 
-script: mvn install
+script: mvn clean install
 env:
   global:
     secure: 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