Switch to container-based infrastructure on Travis CI.

The container-based infrastructure has more memory and more cores.  It also is designed to startup faster. We give up the ability to use sudo, but we aren't using that right now so we should be good.

Props netweb, tw2113.
Fixes #30822.



git-svn-id: https://develop.svn.wordpress.org/trunk@30987 602fd350-edb4-49c9-b593-d223f7449a82
Update npm on Travis CI as a part of build

Travis has an outdated version of NPM.  While this is currently only generating a warning, in the future it could cause actual errors.

Fixes #30823



git-svn-id: https://develop.svn.wordpress.org/trunk@30988 602fd350-edb4-49c9-b593-d223f7449a82