test: use the xvfb-run helper

https://docs.travis-ci.com/user/gui-and-headless-browsers/#using-the-xvfb-run-wrapper

Makefile

@@ -14,13 +14,10 @@ clean:
 lint:
 	flake8 --doctests --exclude=doc
 
-test: export DISPLAY = :99
 test:
-	Xvfb $$DISPLAY -screen 0 800x600x24 &
-	coverage run --branch --source pygsp setup.py test
+	xvfb-run coverage run --branch --source pygsp setup.py test
 	coverage report
 	coverage html
-	killall Xvfb
 
 doc:
 	sphinx-build -b html -d doc/_build/doctrees doc doc/_build/html