Fixes Travis tests to node 4 and 5

pirelenito · pirelenito · commit aa1a3fabd0ba · 2016-02-15T19:54:28.000+01:00
By specifying either “node” or “stable” is failing without a reason.
diff --git a/.travis.yml b/.travis.yml
@@ -1,5 +1,6 @@
 language: node_js
 node_js:
-  - "node"
+  - "4"
+  - "5"
 # send builds to the container-based infrastructure
 sudo: false
