Build: Test on Node.js 16 instead of 15

Node.js 10 is kept for now despite being EOL'd as that's what our current
infrastructure relies on.

Closes gh-4902

Author: mgol

.travis.yml

@@ -1,10 +1,10 @@
 language: node_js
 os: linux
 node_js:
-- "10"
+- "10" # required by jQuery infra
 - "12"
 - "14"
-- "15"
+- "16"
 env:
   - NPM_SCRIPT=test:browserless
 jobs: