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)