tests: use good $DISPLAY

BoboTiG · BoboTiG · commit 62abb826d473 · 2016-10-07T12:43:51.000+02:00
diff --git a/.travis.yml b/.travis.yml
@@ -14,7 +14,7 @@ before_install:
 install:
   - python setup.py install
 
-script: py.test --display=':99'
+script: py.test --display="$DISPLAY"
 
 matrix:
     fast_finish: true
