Move to service instead of involking xvfb directly (#860)

* Move to service instead of involking xvfb directly

* Update .travis.yml

* remove sudo=false

https://blog.travis-ci.com/2018-10-04-combining-linux-infrastructures

Author: ksunden

.travis.yml

@@ -2,11 +2,9 @@ language: python
 matrix:
     include:
         - python: 3.6
-          dist: trusty
-          sudo: false
+          dist: xenial
         - python: 3.7
           dist: xenial
-          sudo: true
 addons:
     apt:
         packages:
@@ -16,8 +14,6 @@ install:
   - pip install cython
   - pip install .
   - pip install -U pytest "coverage<5"
-before_script:
-- "export DISPLAY=:99.0"
-- "sh -e /etc/init.d/xvfb start"
-- sleep 3 # give xvfb some time to start
+services:
+  - xvfb
 script: python setup.py test