test(travis): add node 7 to travis

.travis.yml

@@ -2,8 +2,9 @@ language: node_js
 node_js:
   - "0.10"
   - "0.12"
-  - "4.0"
-  - "6.0"
+  - "4"
+  - "6"
+  - "7"
 
 before_install:
   - 'if [ "$TRAVIS_NODE_VERSION" != "0.10" -a "$TRAVIS_NODE_VERSION" != "0.12" ]; then npm update npm -g; fi'