fix(travis): use new syntax for xenial and pin to it. (#236)

Run latest node 8 and 10.
Commit package-lock.json to lock versions.

.travis.yml

@@ -1,16 +1,20 @@
 sudo: false
+# the xvfb service syntax depends on distro
+dist: xenial
 language: node_js
 node_js:
-  - "8.0"
-  - "10.0"
+  - "8"
+  - "10"
 
 # Make sure we have new NPM.
 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: