compilation with OCaml 4.11

It looks like compilation with OCaml version 4.11.2 errors:
```
ocamlopt -c coq2html.ml
File "coq2html.mll", line 291, characters 5-13:
Error: Unbound value Seq.find
make: *** [Makefile:10: coq2html.cmx] Error 2
```
There is maybe a minimal version for OCaml to require in the README. @yoshihiro503