* ext/psych/lib/psych/exception.rb: there should be only one exception
  base class. Fixes tenderlove/psych #125
* ext/psych/lib/psych.rb: require the correct exception class
* ext/psych/lib/psych/syntax_error.rb: ditto
* ext/psych/lib/psych/visitors/to_ruby.rb: ditto

fixes #125