tool/downloader.rb: Make sure we update to latest version
if network connection is available, but we don't fail if
there is no network connection but option -e is set and
we already have a version of the file.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48053 b2dd03c8-39d4-4d8f-98ff-823fe69b080e