build(ci): Added code coverage reporting using codecov.io

https://codecov.io/gh/okonet/attr-accept

Author: okonet

.travis.yml

@@ -1,7 +1,8 @@
 language: node_js
 node_js:
   - "stable"
+  - "6"
+  - "4"
   - "0.12"
-  - "0.11"
-  - "0.10"
-  - "iojs"
+after_success:
+  - bash <(curl -s https://codecov.io/bash)