Travis run plot tests (#808)

* Travis try to run plot tests

* Setting this to false does nothing so remove it

* dont need explicit install

.travis.yml

@@ -18,20 +18,19 @@ branches:
   only:
     - master
 
-# signal that we are on travis
-# to disable things that expect an
-# X Display
-#
-env:
-    - TRAVISCI=true
 # command to install dependencies
 install:
     - pip install --upgrade pip
     - pip install -r requirements.txt
     - pip install -r test_requirements.txt --upgrade
     - pip install -r docs_requirements.txt
     - pip install -e .
-# command to run tests
+
+before_script: # configure a headless display to test plot generation
+    - "export DISPLAY=:99.0"
+    - "sh -e /etc/init.d/xvfb start"
+    - sleep 3 # give xvfb some time to start
+
 script:
     - cd qcodes
     - py.test --cov=qcodes --cov-report xml --cov-config=.coveragerc