Updates vagrant check to catch Ubuntu 16.04
Updates vagrant check to catch Ubuntu 16.04

The username "vagrant" is standard for VirtualBox vagrant images, but
Ubuntu 16.04 sets the default username to "ubuntu", so let's check both
and skip cloning if either is found.

freedomofpress/fingerprint-securedrop,freedomofpress/FingerprintSecureDrop,freedomofpress/FingerprintSecureDrop,freedomofpress/fingerprint-securedrop,freedomofpress/fingerprint-securedrop