vagrant: add openssl to buildInputs

See inline comment for reasoning; presence of openssl appears to be a
requisite for some libraries to function correctly.

pkgs/development/tools/vagrant/default.nix

@@ -1,5 +1,6 @@
 { stdenv, lib, fetchurl, buildRubyGem, bundlerEnv, ruby, libarchive
 , libguestfs, qemu, writeText, withLibvirt ? stdenv.isLinux
+, openssl
 }:
 
 let
@@ -50,6 +51,11 @@ in buildRubyGem rec {
   dontBuild = false;
   src = fetchurl { inherit url hash; };
 
+  # Some reports indicate that some connection types, particularly
+  # WinRM, suffer from "Digest initialization failed" errors. Adding
+  # openssl as a build input resolves this runtime error.
+  buildInputs = [ openssl ];
+
   patches = [
     ./unofficial-installation-nowarn.patch
     ./use-system-bundler-version.patch