rbinstall.rb: spec date from VCS
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43617 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
rbinstall.rb: spec date from VCS
VCS for the case using git, RUBY_RELEASE_DATE is the last resort.
probably fixes [Bug #9085].
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43617 b2dd03c8-39d4-4d8f-98ff-823fe69b080e