travis: use 'git fetch' to deepen, not 'git pull'

travis: use 'git fetch' to deepen, not 'git pull'

Travis builds of branches (i.e. pull-requests) were testing the wrong
thing, because the 'git pull' was causing current trunk to be
auto-merged. At least that's what seemed to break
https://travis-ci.org/tahoe-lafs/tahoe-lafs/jobs/81517826 : it hung
forever waiting for a commit message to be entered.

Also add "sudo: false" to use travis containers (faster) instead of new
VMs. We only use pip to install dependencies, not apt, so we don't need
root.