Update .travis.yml

.travis.yml

@@ -33,6 +33,8 @@ matrix:
   fast_finish: true
 
 before_install:
+  - "export DISPLAY=:99.0" # needed for testing with `racket/gui`
+  - "sh -e /etc/init.d/xvfb start" # needed for testing with `racket/gui`
   - git clone https://github.com/mbutterick/travis-racket.git
   - cat travis-racket/install-racket.sh | bash # pipe to bash not sh!
   - export PATH="${RACKET_DIR}/bin:${PATH}" #install-racket.sh can't set for us