fix(travis): use new syntax for xenial and pin to it.

Commit package-lock.json to lock versions.

.travis.yml

@@ -1,4 +1,6 @@
 sudo: false
+# the xvfb service syntax depends on distro
+dist: xenial
 language: node_js
 node_js:
   - "8.0"
@@ -8,9 +10,11 @@ node_js:
 before_install:
   - if [[ "`node --version`" = v0* ]] || [[ "`node --version`" = v4* ]]; then npm install -g npm@latest-3; fi
 
+services:
+  - xvfb
+
 before_script:
   - export DISPLAY=:99.0
-  - sh -e /etc/init.d/xvfb start
   - npm install -g grunt-cli
 
 script: