Removed OpenJDK6 from tested platforms on Travis

Travis, openjdk6 and a compatible maven version is just
too much trouble. Easier to move on.

.travis.yml

@@ -4,19 +4,12 @@ addons:
   apt:
     packages:
       - oracle-java8-installer
-      - openjdk-6-jdk
-before_install:
-  - wget https://archive.apache.org/dist/maven/maven-3/3.3.9/binaries/apache-maven-3.3.9-bin.zip
-  - unzip -qq apache-maven-3.3.9-bin.zip
-  - export M2_HOME=$PWD/apache-maven-3.3.9
-  - export PATH=$M2_HOME/bin:$PATH
 install:
   - jdk_switcher use oraclejdk8
   - mvn install -DskipTests=true -Dmaven.javadoc.skip=true
 jdk:
   - oraclejdk8
   - openjdk7
-  - openjdk6
 script:
   - jdk_switcher use $TRAVIS_JDK_VERSION
   - mvn test -Dmaven.javadoc.skip=true