chore(travis): add node 6

Author: wzrdtales

.travis.yml

@@ -6,6 +6,7 @@ node_js:
   - 4.0
   - 4.2
   - 5
+  - 6
   - "iojs-1"
   - "iojs-2"
   - "iojs-3"
@@ -27,6 +28,8 @@ os:
 
 matrix:
   fast_finish: true
+  allow_failures:
+    - node_js: 6
 
 services:
   - mongodb