Fix pytest version (#42)

.travis.yml

@@ -5,7 +5,7 @@ python:
 install:
     - pip install -U pip
     - pip install -r requirements.txt 
-    - pip install pytest pytest-cov pytest-xdist flake8 codecov unittest2
+    - pip install 'pytest>=4.0.2' pytest-cov pytest-xdist flake8 codecov unittest2
 script:
     - make lint
     - make test