vagrant: Windows - Exclude README from ws_migration

Windows doesn't differentiate between files with/without executable bit,
so this can't be used to exclude the README file from the migrations to
be executed, when the sources are accessed from a Windows partition.

rakelib/workspace.rake

@@ -7,10 +7,11 @@ namespace :ws do
     task :migrate => MIGRATION_MARKER_DIR do
         Dir['ws_migrations/*'].select{|m| File.executable?(m)}.each do |migration|
             completion_file = File.join(MIGRATION_MARKER_DIR, File.basename(migration))
-            if ! File.exist?(completion_file)
+            is_excluded = File.basename(migration).start_with?("README")
+            if ! File.exist?(completion_file) && ! is_excluded
                 sh(migration)
                 File.write(completion_file, "")
             end
         end unless SKIP_MIGRATIONS
     end
-end
+end