fix ci

.travis.yml

@@ -4,7 +4,8 @@ python:
 # command to install dependencies
 install:
   - pip install --quiet -r requirements.txt
-  - pip install pytest pytest-cov
+  - pip install pytest>=3.6
+  - pip install pytest-cov
 # command to run tests
 script:
   - pytest --cov=./