modernize travis

Author: dfreedm

.travis.yml

@@ -1,19 +1,18 @@
 language: node_js
 sudo: false
-dist: trusty
-node_js: '9'
+dist: xenial
+node_js: '12'
+servies:
+  - xvfb
 addons:
-  firefox: "66.0"
+  firefox: latest
   chrome: stable
-cache:
-  directories:
-  - node_modules
 before_script:
 - npm install -g gulp-cli@1
 - gulp lint-eslint
 script:
 - node ./node_modules/.bin/polymer test --npm --module-resolution=node -l chrome
-- xvfb-run node ./node_modules/.bin/polymer test --npm --module-resolution=node -l firefox
+- node ./node_modules/.bin/polymer test --npm --module-resolution=node -l firefox
 - if [ "${TRAVIS_PULL_REQUEST}" = "false" ]; then travis_wait 30 ./util/travis-sauce-test.sh; fi
 env:
   global: