Switch from phantomjs to headless chrome

ui/testem.js

@@ -2,6 +2,21 @@
 module.exports = {
   test_page: 'tests/index.html?hidepassed',
   disable_watching: true,
-  launch_in_ci: ['PhantomJS'],
+  launch_in_ci: ['Chrome'],
   launch_in_dev: ['Chrome'],
+  browser_args: {
+    // New format in testem/master, but not in a release yet
+    // Chrome: {
+    //   ci: ['--headless', '--disable-gpu', '--remote-debugging-port=9222', '--window-size=1440,900'],
+    // },
+    Chrome: {
+      mode: 'ci',
+      args: [
+        '--headless',
+        '--disable-gpu',
+        '--remote-debugging-port=9222',
+        '--window-size=1440,900',
+      ],
+    },
+  },
 };