Try using xvfb as a service (to fix Travis CI failures)

.travis.yml

@@ -17,6 +17,8 @@ addons:
       - liblapack-dev
     config:
       retries: true
+services:
+  - xvfb
 cache:
   directories:
   - $HOME/nndc_hdf5
@@ -31,7 +33,6 @@ env:
     - OPENMC_ENDF_DATA=$HOME/endf-b-vii.1
     - LD_LIBRARY_PATH=$HOME/MOAB/lib:$HOME/DAGMC/lib
     - PATH=$PATH:$HOME/NJOY2016/build
-    - DISPLAY=:99.0
     - COVERALLS_PARALLEL=true
   matrix:
     - OMP=n MPI=n PHDF5=n

tools/ci/travis-before-script.sh

@@ -1,9 +1,5 @@
 #!/bin/bash
 set -ex
 
-# Allow tests that require GUI as described at:
-# https://docs.travis-ci.com/user/gui-and-headless-browsers/#Using-xvfb-to-Run-Tests-That-Require-a-GUI
-sh -e /etc/init.d/xvfb start
-
 # Download NNDC HDF5 data, ENDF/B-VII.1 distribution, multipole library
 source tools/ci/download-xs.sh