* ext/psych/lib/psych/scalar_scanner.rb: fix support for negative
  years.
* ext/psych/lib/psych/visitors/yaml_tree.rb: ditto
* test/psych/test_date_time.rb: test for change.
  Fixes: https://github.com/tenderlove/psych/issues/168

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43864 b2dd03c8-39d4-4d8f-98ff-823fe69b080e