Update build to only test on supported versions of node.

- Remove <= 7 as they're now (or about to be) out of LTS.

Author: Codex-

.travis.yml

@@ -1,13 +1,10 @@
 sudo: false
 language: node_js
 node_js:
+  - "11"
   - "10"
   - "9"
   - "8"
-  - "7"
-  - "6"
-  - "5"
-  - "4"
 
 services:
   - docker