Pin OCamlformat version in CI so that we can invalidate the cache (#45)

Author: anmonteiro

.circleci/config.yml

@@ -31,7 +31,7 @@ opam-steps: &opam-steps
           sudo apt-get install -y pkg-config libncurses5-dev
           opam update
           opam install -y dune
-          opam install -y ocamlformat
+          opam install -y ocamlformat.0.12
     - run:
         name: 'Clean'
         command: rm -rf ./_build