Make yaml to handle anchor aliased class correctly

Currently, since `psych_ext` is based on old `psych`, aliased class is not loaded correctly.
See example below:

```
$ pry 
[1] pry(main)> require 'yaml'
=> true
[2] pry(main)> YAML.load("created_at: &1 2014-02-19 15:19:41.842913000 +09:00\nupdated_at: *1\n")
=> {"created_at"=>2014-02-19 15:19:41 +0900,
 "updated_at"=>2014-02-19 15:19:41 +0900}
[3] pry(main)> require 'delayed_job'
=> true
[4] pry(main)> YAML.load("created_at: &1 2014-02-19 15:19:41.842913000 +09:00\nupdated_at: *1\n")
=> {"created_at"=>2014-02-19 15:19:41 +0900,
 "updated_at"=>"2014-02-19 15:19:41.842913000 +09:00"}
```

I know edited lines are solving the issue of default implementation change of YAML during ruby 1.9.2/1.9.3 generation. https://github.com/collectiveidea/delayed_job/commit/14cfa873f85bfe24459202e461512216144adbce

But current code of psych does not seem to cause such issue. Please refer the current psych and review this pull request :-)

Current source of psych:
https://github.com/tenderlove/psych/blob/master/lib/psych/visitors/yaml_tree.rb
https://github.com/tenderlove/psych/blob/master/lib/psych/visitors/to_ruby.rb