Fix the ott version to be at most 0.30

Version 0.31 breaks our fragile usage of Ott.
5 jobs for master in 16 minutes and 30 seconds (queued for 1 second)
latest
Status Job ID Name Coverage
  Build
passed #740447614
coq

00:03:45

17.92%
passed #621518892
coq:8.8

00:10:15

passed #621518895
coq:8.9

00:09:42

passed #621518899
publish:docker:master

00:12:44

passed #621518897
coq

00:09:35

18.71%