Dirty fix for berkshelf#88

Works for me, on OS X 10.9 with vagrant-vmware-fusion.

To install, checkout this branch and run:
    bundle install
    bundle exec thor :build
    vagrant plugin uninstall vagrant-berkshelf
    vagrant plugin install pkg/vagrant-berkshelf-1.3.7.gem

lib/berkshelf/vagrant/action/install.rb

@@ -40,10 +40,22 @@ def call(env)
          def install(env)
            check_vagrant_version(env)
            env[:berkshelf].ui.info "Updating Vagrant's berkshelf: '#{env[:berkshelf].shelf}'"

            # https://github.com/berkshelf/vagrant-berkshelf/issues/88
            # Some of Vagrant's folder sharing methods die when the
            # shared folder is deleted and recreated, as Berkshelf
            # does.  To work around this, we install to a temporary
            # location, then use rsync to update the directory shared
            # by Vagrant.
            real_shelf = env[:berkshelf].shelf
            tmp_shelf = "#{real_shelf}-tmp"

            opts = {
              path: env[:berkshelf].shelf
              path: tmp_shelf
            }.merge(env[:global_config].berkshelf.to_hash).symbolize_keys!
            env[:berkshelf].berksfile.install(opts)

            system("rsync -aW --delete #{tmp_shelf}/. #{real_shelf}/.")
          end

          def warn_disabled_but_berksfile_exists(env)