Update travis congig.

.travis.yml

@@ -1,6 +1,16 @@
 language: node_js
 node_js:
-    - "0.12"
+  - "0.12"
+  - "4"
+env:
+  - CXX="g++-4.8"
+addons:
+  apt:
+    sources:
+      - ubuntu-toolchain-r-test
+    packages:
+      - g++-4.8
+      - gcc-4.8
 before_install:
   - sudo apt-get update
   - sudo apt-get install libzmq3-dev