* tool/rbinstall.rb: always require rubygems to stabilize rubygems
  related status like whether Gem::Specification is defined or not.

* tool/rbinstall.rb (Gem::Specification.unresolved_deps): define stub.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41877 b2dd03c8-39d4-4d8f-98ff-823fe69b080e