9.4.1 -> 9.4.2 problem: could not access status of transaction 1

I have a database that was upgraded from 9.4.1 to 9.4.2 (no pg_upgrade, we
just dropped new binaries in place) but it wouldn't start up. I found this
in the logs:

waiting for server to start....2015-05-27 13:13:00 PDT [27341]: [1-1] LOG:
database system was shut down at 2015-05-27 13:12:55 PDT
2015-05-27 13:13:00 PDT [27342]: [1-1] FATAL: the database system is
starting up
.2015-05-27 13:13:00 PDT [27341]: [2-1] FATAL: could not access status of
transaction 1
2015-05-27 13:13:00 PDT [27341]: [3-1] DETAIL: Could not open file
"pg_multixact/offsets/0000": No such file or directory.
2015-05-27 13:13:00 PDT [27340]: [1-1] LOG: startup process (PID 27341)
exited with exit code 1
2015-05-27 13:13:00 PDT [27340]: [2-1] LOG: aborting startup due to
startup process failure
stopped waiting

I poked around in pg_multixact/offsets and there are lots of other files in
there, just not 0000.

I tried launching postgres in standalone mode to try some debugging but it
yields the same error.

AFAICT, there's nothing wrong with hardware, it's a VM, and it's been
working okay. About a week ago we upgraded it from 9.3.5 to 9.4.1 via
pg_upgrade --link, which worked amazingly well.

I found [this report from a couple days ago](
https://bugs.archlinux.org/task/45071) from someone else that looks like
the same problem.

I put this box's [postgresql.conf up on a gist](
https://gist.github.com/skehlet/3589b0d83f2cafe19624).

What can I try next? Thanks!