fix travis conf file

Author: ernestoarbitrio

.travis.yml

@@ -10,8 +10,8 @@ python:
 
 # command to install dependencies
 install:
-  - pip install -r requirements.txt
-  - pip install -r requirements-dev.txt
+  - python -m pip install -e .[testing]
+  - python -m pip install flake8
 
 # command to run tests
 script: