fix(chore) Fix the travis CI config (#50)

Author: Clément Denoix

.travis.yml

@@ -1,3 +1,7 @@
+dist: xenial
+services:
+  - xvfb
+
 env:
   global:
   - CODECLIMATE_REPO_TOKEN=a47935830d841ad61a6e960be8a3b6a5e557146ac010dafa993e61bf82898472
@@ -19,8 +23,6 @@ before_script:
   - tar -xzf geckodriver-v0.11.1-linux64.tar.gz -C geckodriver
   - export PATH=$PATH:$PWD/geckodriver
   - "export DISPLAY=:99.0"
-  - "sh -e /etc/init.d/xvfb start"
-  - sleep 3
 
 script:
 - pytest --cov-config .coveragerc --cov=scrapy_selenium tests/