Try expanding virtual screen to fix Travis.

.travis.yml

@@ -5,19 +5,16 @@ node_js:
 cache:
   directories:
     - "node_modules"
+    - "$HOME/.cache/pip"
 
 sudo: required
 addons:
   chrome: stable
-
+ 
 before_script:
- - "export DISPLAY=:99.0"
- - "sh -e /etc/init.d/xvfb start"
- - sleep 3 # give xvfb some time to start
-
  - google-chrome-stable --headless --disable-gpu --remote-debugging-port=9222 http://localhost &
  - pip install --user python-igraph
     
 script:
- - ./run_tests.sh
+ - xvfb-run --server-args="-screen 0 1440x900x24" ./run_tests.sh