DEV: use tox instead of plain py.test

.gitlab-ci.yml

@@ -86,13 +86,13 @@ test:pycaosdb:py34:
     tags: [ py34 ]
     stage: test
     script:
-        - py.test-3.4
+        - tox -r -e py34
 
 test:pycaosdb:py27:
     tags: [ py27 ]
     stage: test
     script:
-        - py.test-2.7
+        - tox -r -e py27
 
 # pylint tests for pycaosdb (python 3.4)
 .test:pycaosdb:python3.4:pylint: