Allow tests to fail on latest node version

I'm not sure this works or if you have to specify an exact version
for "allow_failures". Anyway, this is just a temporary fix until #24 has
been solved.

Also cleaned up the version we test against:
* 5.x and 7.x removed as their EOL was at 2016-06-30
* 8.x is the current active LTS release

(see https://github.com/nodejs/Release)

.travis.yml

@@ -2,10 +2,12 @@ language: node_js
 node_js:
   - "0.12"
   - 4    # latest 4.x release
-  - 5    # latest 5.x release
   - 6    # latest 6.x release
-  - 7    # latest 7.x release
+  - 8    # latest 8.x release
   - node # latest stable Node.js release
+matrix:
+  allow_failures:
+    - node_js: node
 sudo: false
 notifications:
   email: false