tox: Update xvfbwrapper to 0.2.8.

- better error handling when xvfb is not installed
- don't hardcode tmp dir location
- use terminate instead of kill so Xvfb can clean up lock files

tox.ini

@@ -42,7 +42,7 @@ deps =
     vulture==0.8.1
     Werkzeug==0.11.3
     wheel==0.26.0
-    xvfbwrapper==0.2.7
+    xvfbwrapper==0.2.8
     cherrypy==4.0.0
 commands =
     {envpython} scripts/link_pyqt.py --tox {envdir}