Changes

osmo-gsm-tester: fix getting latest commit

Get the latest commit from gerrit.osmocom.org, because after we moved
repositories from git.osmocom.org it currently answers with a 404.

Related: https://osmocom.org/projects/cellular-infrastructure/wiki/Git_infrastructure
Change-Id: I0e2215d067dec1e2327f765b5a97ba7512812647