Install dev requirements to run the tests.

Author: mithrandi

.travis.yml

@@ -11,7 +11,7 @@ python:
   - "pypy3"
 
 install:
-  - travis_retry pip install -r requirements.txt
+  - travis_retry pip install -r requirements.txt -r requirements-dev.txt
   - if [ "$TRAVIS_PYTHON_VERSION" == "3.2" ]; then travis_retry pip install 'coverage<4'; fi
   - travis_retry pip install coveralls