rbinstall.rb: add date

* tool/rbinstall.rb (Gem::Specification#initialize): default date to
  RUBY_RELEASE_DATE.  [ruby-core:57072] [Bug #8878]
* tool/rbinstall.rb (Gem::Specification#to_ruby): add date.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42890 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2013-09-09

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42891 b2dd03c8-39d4-4d8f-98ff-823fe69b080e