vagrant: update deps for 2.1.2

The version was bumped but the dependencies were not updated.
Do so and add some instructions for future package updates.

pkgs/development/tools/vagrant/default.nix

@@ -1,6 +1,10 @@
 { lib, fetchurl, buildRubyGem, bundlerEnv, ruby, libarchive }:
 
 let
+  # NOTE: bumping the version and updating the hash is insufficient;
+  # you must copy a fresh Gemfile.lock from the vagrant source,
+  # and use bundix to generate a new gemset.nix.
+  # Do not change the existing Gemfile.
   version = "2.1.2";
   url = "https://github.com/hashicorp/vagrant/archive/v${version}.tar.gz";
   sha256 = "0fb90v43d30whhyjlgb9mmy93ccbpr01pz97kp5hrg3wfd7703b1";