Add gitlab testing.

Have chkcrontab be tested first at home.

.gitlab-ci.yml

+before_script:
+  - python setup.py install
+
+test:2.7:
+  image: python:2.7-alpine
+  type: test
+  script:
+    - python setup.py test
+
+test:3.3:
+  image: python:3.3-alpine
+  type: test
+  script:
+    - python setup.py test
+
+test:3.4:
+  image: python:3.4-alpine
+  type: test
+  script:
+    - python setup.py test
+
+test:3.5:
+  image: python:3.5-alpine
+  type: test
+  script:
+    - python setup.py test
+
+test:3.6:
+  image: python:3.6-alpine
+  type: test
+  script:
+    - python setup.py test