Modify Travis CI config to get python-control tests to pass

Author: roryyorke

.travis.yml

@@ -1,4 +1,9 @@
 # Travis configuration file for slycot
+dist: xenial
+
+services:
+  - xvfb
+
 matrix:
   include:
     - name: "Python 2.7, TEST_CONDA=0"
@@ -95,9 +100,6 @@ script:
   #
   # Additional packages required for python-control
   - conda install scipy matplotlib
-  # Install display manager to allow testing of plotting functions
-  - export DISPLAY=:99.0
-  - sh -e /etc/init.d/xvfb start
   # Get python-control from source and install
   - git clone https://github.com/python-control/python-control.git control
   - cd control; python setup.py test