chore: Change travis.yml to list supported node versions

Author: Sander Beekhuis

.travis.yml

@@ -1,8 +1,9 @@
 language: node_js
 node_js:
-  - 6 # to be removed 2019-04-01
-  - 8 # to be removed 2019-12-31
-  - 9 # to be removed 2018-06-30
+  - 8 # LTS to be removed 2019-12-31
+  - 10 # LTS to be removed 2021-04-30
+  - 11 # to be removed 2019-06-01
+  - 12 # future LTS to be removed 2022-04-30
   - lts/* # safety net; don't remove
   - node # safety net; don't remove
 deploy: