Update .travis.yml

.travis.yml

@@ -5,7 +5,7 @@ python:
 # command to install dependencies
 install: 
   - "python setup.py -q install"
-  - "pip install pytest-pep8 pytest-cov codecov"
+  - "pip install --upgrade pytest pytest-pep8 pytest-cov codecov"
 # command to run tests
 script:
   - "pytest --cov-config .coveragerc --cov=./"