fix xvfb in travis

Author: gus-massa

.travis.yml

@@ -5,9 +5,10 @@ language: c
 env:
 - PATH=~/racket/bin:$PATH
 
+services:
+- xvfb
+
 before_install:
-- "export DISPLAY=:99.0"
-- "sh -e /etc/init.d/xvfb start"
 #- curl -L -o installer.sh http://plt.eecs.northwestern.edu/snapshots/current/installers/racket-test-current-x86_64-linux-precise.sh
 - curl -L -o installer.sh http://www.cs.utah.edu/plt/snapshots/current/installers/racket-current-x86_64-linux-precise.sh
 - sh installer.sh --in-place --dest ~/racket/