Update nodejs versions used for Travis-CI tests

Node 4 and 5 are now obsolete, and so there's no need to run tests against them.

Versions 6, 8 and 10 are still within support as of this commit, with 6 in LTS maintenance and 8 and 10 under active development. Node 9 will be obsolete in a few weeks, so we'll just skip adding it in the first place.

Author: apparentlymart

.travis.yml

@@ -1,6 +1,6 @@
 language: node_js
 node_js:
-  - "4"
-  - "5"
   - "6"
+  - "8"
+  - "10"
 script: npm test && npm run-script lint