Update travis.yml to fix failing tests

Due to the latest seen error message from Travis-CI

```bash
ERROR: npm is known not to run on Node.js v4.0.0
Node.js 4 is supported but the specific version you're running has
a bug known to break npm. Please update to at least ${rel.min} to use this
version of npm. You can find the latest release of Node.js at https://nodejs.org/
```
the node.js version of the 4.x release cycle was updated to the latest `4.8.4` release.

.travis.yml

@@ -3,7 +3,7 @@ language: node_js
 notifications:
   email: false
 node_js:
-  - 4.0
+  - 4.8.4
   - 6.9.4
   - 7.4.0
   - node