Travis: ditch ancient before_install hook that upgrades npm and cleans its cache
Travis: ditch ancient before_install hook that upgrades npm and cleans its cache

It that seems to be causing trouble now because the latest npm does not
support the oldest node versions we support, so let's see if we can
avoid it altogether.

https://travis-ci.org/unexpectedjs/unexpected-sinon/jobs/250276587#L141-L147

unexpectedjs/unexpected-sinon,unexpectedjs/unexpected-sinon