use Chrome for testing headless

Author: Anthony Kirwan

testem.js

@@ -4,10 +4,17 @@ module.exports = {
   "test_page": "tests/index.html?hidepassed",
   "disable_watching": true,
   "launch_in_ci": [
-    "PhantomJS"
+    "Chrome"
   ],
   "launch_in_dev": [
-    "PhantomJS",
     "Chrome"
-  ]
+  ],
+  "browser_args": {
+    Chrome: [
+      '--disable-gpu',
+      '--headless',
+      '--remote-debugging-port=9222',
+      '--window-size=1440,900'
+    ]
+  }
 };