Add support for checking module on python3, like on core (#2235)

lib/ansible/modules/extras/.travis.yml

@@ -9,6 +9,7 @@ addons:
     packages:
       - python2.4
       - python2.6
+      - python3.5
 before_install:
   - git config user.name "ansible"
   - git config user.email "ansible@ansible.com"
@@ -20,5 +21,7 @@ script:
   - python2.4 -m compileall -fq -x 'cloud/|monitoring/zabbix.*\.py|/dnf\.py|/layman\.py|/maven_artifact\.py|clustering/(consul.*|znode)\.py|notification/pushbullet\.py|database/influxdb/influxdb.*\.py' .
   - python2.6 -m compileall -fq .
   - python2.7 -m compileall -fq .
+  - python3.4 -m compileall -fq system/at.py
+  - python3.5 -m compileall -fq system/at.py
   - ansible-validate-modules .
   #- ./test-docs.sh extras