* tool/rbinstall.rb (Gem::Specification.last_date): skip if failed to
  get info from VCS, for example, in the case circumstance sharing working
  directory with another machine and run vcs tools cannot work.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43624 b2dd03c8-39d4-4d8f-98ff-823fe69b080e