Remove psych patch for ruby 1.9.3 p194 since it's been long fixed.

The String hack is only needed on patch level 194 since it's fixed here: https://github.com/tenderlove/psych/commit/620fc6d749f0e94f7b433af9419927039ca1bfa4

For reference, the remaining code is required since:
The outstanding issue with serializing Hash subclasses is still an open issue here: https://github.com/tenderlove/psych/issues/43