add upgrade flag to dev requirements

Author: jessecooper

.travis.yml

@@ -5,7 +5,7 @@ python:
 - '3.8'
 install:
 - pip install -r requirements.txt
-- pip install -r dev_requirements.txt
+- pip install -r dev_requirements.txt --upgrade
 - pip install coveralls
 script:
 - pytest --cov