Auto merge of #4991 - bennofs:master, r=matklad

Require at least version 0.1.9 of jobserver crate

There are some important fixes in jobserver >=0.1.8. With earlier
versions, it's possible for cargo to panic with a "failed to acquire
jobserver token" error, which can be very hard to track down.

Requiring the latest version of jobserver makes sure that no such error
can make it into downstream distributions.