Fix tests on Linux

[beajeanm]

• The pin of reason is no longer necessary as reason 3.8 supports ocaml 4.13
• Fix a higher bond to caqti. Caqti 1.8 introduces new deprecations that would require code change.
• The behavior of Printexec.to_string (Failure "msg") has changed from 4.09 to 4.10. It used to be (Failure "write to a read-only stream") but it's now Failure ("write to a read-only stream"). Since we already know the exception's type, I figure it would be enough to match on the message itself.