travis: Install dev dependencies

Author: tony

.travis.yml

@@ -37,6 +37,7 @@ cache:
 install:
   - pip install -e .
   - pip install -r requirements/test.txt
+  - pip install -r requirements/dev.txt
 before_script:
   - mkdir -p ~/source
   - mkdir -p ~/build/tmux-$TMUX_VERSION