Update version of Coq (8.7.1)

README.md

@@ -30,7 +30,7 @@ To build Q\*cert from the source, you will need:
   - camlp5, a pre-processor (https://camlp5.github.io)
   - base64, a library for base64 encoding and decoding (https://github.com/mirage/ocaml-base64)
   - js\_of\_ocaml, a compiler from OCaml to JavaScript
-- Coq 8.7.0 (https://coq.inria.fr/)
+- Coq 8.7.1 (https://coq.inria.fr/)
 
 An easy way to get set up on most platforms is to use the OCaml
 package manager (https://opam.ocaml.org). Once opam is installed, you