chore(travis): update Node versions tested

Synchronise with Karma itself, but retaining `stable` for now. This
should unbreak the repository so that other patches can be merged; right
now, the master branch fails Travis CI tests, due these EOLed build targets.

Closes #260.

Author: jdforrester

.travis.yml

@@ -1,11 +1,10 @@
 sudo: false
 language: node_js
 node_js:
+  - 6
+  - 8
+  - 10
   - stable
-  - 5
-  - 4
-  - "0.12"
-  - "0.10"
 
 # Make sure we have new NPM.
 before_install: