Indicate proper required ruby version in Gem specification

Hello,

As Rails 3.0 beta now depends on a minimum version of Ruby (1.8.7) and since support for previous versions has been deprecated, will be great that the gem specification used to build and deliver Rails and it's libraries properly states it.

Current gem specification:

required_ruby_version: !ruby/object:Gem::Requirement
  requirements:
  - - ">="
    - !ruby/object:Gem::Version
      version: "0"
  version:

Which allow installation of Rails 3.0 in older versions of Ruby.

Please refer to the RubyGems documentation to indicate the version constrain:

http://docs.rubygems.org/read/chapter/20#required_ruby_version

Thank you.