remove the extra install of flake8

.travis.yml

@@ -32,12 +32,10 @@ before_script:
 # Package installation
 install:
   - pip install codecov
+
   # Install project dependencies
   - pip install -r requirements.txt
 
-  # Install test dependencies
-  - pip install flake8
-
   # Install node
   - nvm install 8