git: constraint to cstruct <3.1.1 due to ocplib-endian dep

[avsm]

This is because cstruct 3.2.0 will remove ocplib-endian use
(mirage/ocaml-cstruct#177), which in turn causes a transitive
build failure on older versions of git (mirage/ocaml-git#233)

[camelus]

✅ All lint checks passed 29f69b7

• These packages passed lint tests: git.1.6.0, git.1.6.1, git.1.6.2, git.1.7.0, git.1.7.1, git.1.7.2, git.1.8.0, git.1.9.0, git.1.9.1, git.1.9.2, git.1.9.3

---

✅ Installability check (7503 → 7503)

[seliopou]

Looks like all the build failures were caused by the source code for findlib being unavailable. I restarted the datakit jobs but Travis needs a kick as well.

[dbuenzli]

Travis kicked !