bonfire: fix build

The requirement's constraint for `pythonPackages.keyring` didn't allow
keyring v11. However it has been bumped in 93a16a2ace93fd96866d98181a07a8c5a779acce
by @FRidh.

Current failure: https://nix-cache.s3.amazonaws.com/log/2nw5adfx86jwiax32mn11kqpc35xwhsh-bonfire-unstable-2017-01-19.drv
See ticket #36453