common.mk: verconf.h dependency

* common.mk (verconf.h): no needs to update always unless rbconfig.rb
  is updated.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40771 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
yaml_tree.rb: fix warning message

* ext/psych/lib/psych/visitors/yaml_tree.rb: fix warning message.
  Class.create does not exist.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40772 b2dd03c8-39d4-4d8f-98ff-823fe69b080e