BUG: test different python versions

.gitlab-ci.yml

@@ -5,6 +5,17 @@ before_script:
   - pip install .
   - pip install pytest
 
-test:
+test36:
+  image: python:3.6
   script:
     - pytest gyral_structure/test
+
+test35:
+  image: python:3.5
+  script:
+  - pytest gyral_structure/test
+
+test27:
+  image: python:2.7
+  script:
+  - pytest gyral_structure/test