common.mk: update unicode files every times

* common.mk (update-unicode): remove -e option to force update
  every builds.

* tool/downloader.rb (Downloader.download): return if ims is nil
  and target file is existing.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48056 b2dd03c8-39d4-4d8f-98ff-823fe69b080e