.travis.yml: Tweak Node.js versions

Do not test under io.js (which is no longer a thing), test under Node.js
4.0.x and 5.0.x.

Zero minor versions are intentional, I wan't to make sure that PEG.js
doesn't depend on any features added later in the 4.x and 5.x cycle.

.travis.yml

@@ -3,7 +3,8 @@ language: node_js
 node_js:
   - "0.10"
   - "0.12"
-  - "iojs"
+  - "4.0"
+  - "5.0"
 cache:
   directories:
     - node_modules