.travis.yml: Update

Remove deprecated sudo, update Node.js versions to current actively
maintained and LTS versions, use Ubuntu 18.04 "Bionic" environment
instead of 16.04 "Xenial".

.travis.yml

@@ -1,10 +1,10 @@
 language: node_js
 node_js:
-  - 8
   - 10
-  - 11
+  - 12
+  - 13
+  - 14
 script: npm run build
-sudo: false
-dist: xenial
+dist: bionic
 
 # vim: ts=2 sw=2 et