Move Travis to Xenial

For some reason this seems to have started happening anyway (we currently
use the default), and it'd be better to do it explicitly. Also lets us
simplify our xvfb setup somewhat.

Author: pimterry

.travis.yml

@@ -1,3 +1,4 @@
+dist: xenial
 sudo: required
 language: node_js
 node_js:
@@ -9,11 +10,11 @@ before_install:
 install:
 - npm install --global npm@4
 - npm install
+services:
+- xvfb
 before_script:
 - sudo chown root /opt/google/chrome/chrome-sandbox
 - sudo chmod 4755 /opt/google/chrome/chrome-sandbox
-- export DISPLAY=:99.0
-- sh -e /etc/init.d/xvfb start
 script:
 - npm test
 - npm run build:src