common.mk: install-gem

* common.mk (install-gem): target to install default gems only.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@38865 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2013-01-18

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@38866 b2dd03c8-39d4-4d8f-98ff-823fe69b080e