circleci: use a patched opam.rb

The current one gives the following error:

    Error: opam: "cxx11" is not a recognized standard

I cargo culted this change from moby/vpnkit@4bb313b

Signed-off-by: Ian Campbell <ijc@docker.com>

Author: Ian Campbell

.circleci/config.yml

@@ -43,7 +43,7 @@ jobs:
       # until the base image is updated.
       - run: brew upgrade python
       # Install opam 1 until we become opam 2 compatible
-      - run: brew install https://raw.githubusercontent.com/Homebrew/homebrew-core/72ce8812eaa33abe23533dfa021b51351a6b9c3e/Formula/opam.rb
+      - run: brew install https://gist.githubusercontent.com/djs55/7a94ee5aeb882ef5399c0485d2affdda/raw/bc04ff96e0082d7ee07642337dbb77c51b93d678/opam.rb
       - run: opam init -v -n --comp="${OPAM_COMP}" --switch="${OPAM_COMP}" local "${OPAM_REPO}"
       - run: opam config exec -- opam depext -i hyperkit
       - run: opam config exec -- make clean