ocamlPackages.bitstring: 2.1.0 -> 2.1.1

Semi-automatic update generated by https://github.com/ryantm/nix-update tools. These checks were done:

- built on NixOS
- Warning: no binary found that responded to help or version flags. (This warning appears even if the package isn't expected to have binaries.)
- found 2.1.1 with grep in /nix/store/82mvh5s9b7njvwvf2a0c3bw27qm0af4n-ocaml-bitstring-2.1.1
- directory tree listing: https://gist.github.com/6a7bc29539027df0f0f45f8f9e9c1b45