removed useless xvfb setup

.travis.yml

@@ -1,15 +1,14 @@
 language: python
+
 python:
   - 3.6
   - nightly
-before_install:
-  - "export DISPLAY=:99.0"
-  - "sh -e /etc/init.d/xvfb start"
-# command to install dependencies, e.g. pip install -r requirements.txt --use-mirrors
+
 install: 
   - pip install .
   - pip install nose coverage
   - pip install coveralls
+
 # # command to run tests, e.g. python setup.py test
 script:
   - python setup.py nosetests --with-coverage --cover-package astar