common.mk: ims=nil option

* common.mk (update-gems): use ims=nil option.

* tool/downloader.rb (Downloader.download): show the message if no
  need to download in verbose mode.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48497 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2014-11-20

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48498 b2dd03c8-39d4-4d8f-98ff-823fe69b080e