Build: Resolve Travis config warnings

Travis reports warnings in our config:
* root: deprecated key sudo (The key `sudo` has no effect anymore.)
* root: missing os, using the default linux
* root: key matrix is an alias for jobs, using jobs

They are all now resolved.

Closes gh-4636

(cherry picked from commit 5b94a4f847)

.travis.yml

@@ -1,5 +1,5 @@
 language: node_js
-sudo: false
+os: linux
 node_js:
 - "8"
 - "10"
@@ -7,7 +7,7 @@ node_js:
 - "13"
 env:
   - NPM_SCRIPT=test:browserless
-matrix:
+jobs:
   include:
     - name: "Browser tests: full build, Chrome & Firefox stable"
       node_js: "12"