Add syntax checker for YAML using Ruby (MRI)

I fix the following things according to your suggestions In previous PR #235.

1. update the manual in `flycheck.texi`
2. rename the syntax checker `yaml` to `yaml-ruby`
3. support installed Ruby version 1.9.2 or lower
4. Remove the second `(zero-or-more not-newline)` from Psych error pattern

In previous PR #235, I note that I will change the Ruby code, but I add a pattern for both error formats to the syntax checker after all. Because changed Ruby code is too little complicated to run Ruby 'one liner'.