Add Node v4 and remove iojs

Since 3.x will likely be the last line of iojs, and Node v4 is LTS-destined. I imagine the entry "4" will continue to be the latest stable release of the 4.x line on travis and with the help of semver should represent compatibility on all of 4.x 😄

Author: wbinnssmith

.travis.yml

@@ -2,4 +2,4 @@ language: node_js
 node_js:
   - "0.10"
   - "0.12"
-  - "iojs"
+  - "4"