vcs.rb: make Time with proper offset

* tool/vcs.rb (get_revisions): use Time.new instead of Time.mktime
  which does not accept UTC offset, and offset manually for older
  versions than 1.9.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48559 b2dd03c8-39d4-4d8f-98ff-823fe69b080e