chore(travis): simplify and improve config

Includes the following changes:
- Run tests on macOS and Windows as well.
- Run tests against the current Node.js LTS version (v10.x).
- Cache npm cache directory between builds.

Author: gkalpak

.travis.yml

@@ -1,21 +1,23 @@
-sudo: required
 dist: trusty
 
 language: node_js
 node_js:
-  - 6
+  - 10
 
-install:
-  - export DISPLAY=:99.0
+addons:
+  chrome: stable
+
+cache:
+  directories:
+    - "$HOME/.npm"
+
+before_install:
+  - export DISPLAY=":99.0"
   - sh -e /etc/init.d/xvfb start
-  - wget -q -O - https://dl-ssl.google.com/linux/linux_signing_key.pub | sudo apt-key add -
-  - sudo sh -c 'echo "deb [arch=amd64] http://dl.google.com/linux/chrome/deb/ stable main" >> /etc/apt/sources.list.d/google-chrome.list'
-  - sudo apt-get update -q
-  - sudo apt-get install -q google-chrome-stable
 
-before_script:
+install:
   - npm install
 
 script:
   - npm run test-single-run
-  - (npm start > /dev/null &) && (npm run protractor)
+  - (npm start > /dev/null &) && npm run protractor