Chore - Remove Node v7 from CI, only keep LTS versions and Node v9. (#370)

* chore(.travis.yml) node 7 removed from CI, yaml indented

* chore(.travis.yml) run CI only on node LTS versions

* chore(.travis) node v9 added

* chore(.travis) node v4 removed

.travis.yml

@@ -1,11 +1,9 @@
 language: node_js
 sudo: false
 node_js:
-- '4'
-- '6'
-- '7'
-- '8'
-- '9'
+  - 6
+  - 8
+  - 9
 script:
   - npm run ci
 env: