Add browser tests via karma

.travis.yml

@@ -1,3 +1,4 @@
+sudo: false
 language: node_js
 node_js:
   - "5"
@@ -6,10 +7,14 @@ node_js:
   - "8"
   - "node"
 
-sudo: false
+addons:
+  chrome: stable
+  firefox: latest
+
 script:
   - npm test
   - npm run lint
+  - npm run browser-test
 
 git:
   depth: 1