Fix tests (#96)

Closes #63

- Switch from testling to jsdom for browser APIs
- Switch from coverify to nyc for coverage reports
- Clean up related dead code and tooling
- Update Travis to use Node v6 and v8, since we need ES6 features for jsdom.

.travis.yml

@@ -1,7 +1,4 @@
 language: "node_js"
-before_script:
-  # testling use headless browser
-  # on travis-ci, firefox is the default one
-  # & it needs a display to works
-  - export DISPLAY=:99.0
-  - sh -e /etc/init.d/xvfb start
+node_js:
+  - "6"
+  - "8"