Configure travis to execute selenium tests

.travis.yml

@@ -3,13 +3,18 @@ language: python
 python:
     - "2.7"
 env:
-    - CKANVERSION=2.6.4 POSTGISVERSION=2
     - CKANVERSION=2.7.2 POSTGISVERSION=2
 services:
     - redis-server
     - postgresql
+addons:
+    firefox: "46.0"
 install:
     - bash bin/travis-build.bash
+before_script:
+  - "export DISPLAY=:99.0"
+  - "sh -e /etc/init.d/xvfb start"
+  - sleep 3 # give xvfb some time to start
 script:
     - sh bin/travis-run.sh
 after_success: coveralls