add missing headless chrome setup for Travis (#2807)

.travis.yml

@@ -85,7 +85,5 @@ after_success:
 
 cache:
   directories:
-  - ./node_modules/material-design-lite
-  - ./node_modules/material-design-icons
   - demo-shell/node_modules
   - lib/node_modules

lib/config/karma.conf-all.js

@@ -76,6 +76,18 @@ module.exports = function (config) {
 
         browsers: ['Chrome'],
 
+        customLaunchers: {
+            ChromeHeadless: {
+                base: 'Chrome',
+                flags: [
+                    '--no-sandbox',
+                    '--headless',
+                    '--disable-gpu',
+                    '--remote-debugging-port=9222'
+              ]
+            }
+        },
+
         // Karma plugins loaded
         plugins: [
             require('../node_modules/karma-jasmine'),