Add quotes to the strings "y" and "n"

[tenderlove]

'y' and 'n' are kind of ambiguous. Syck treated y and n literals in
YAML documents as strings. But this is not what the YAML 1.1 spec says.
YAML 1.1 says they should be treated as booleans. When we're dumping
documents, we know it's a string, so adding quotes will eliminate the
"ambiguity" in the emitted document

Fixes #443

[ColinDKelley]

@tenderlove It looks like this is still broken for "Y" and "N".

Taken from Regexp given in the Boolean Language-Independent Type for YAML™ Version 1.1:

irb> yaml_1_1_bools = "y|Y|yes|Yes|YES|n|N|no|No|NO
|true|True|TRUE|false|False|FALSE
|on|On|ON|off|Off|OFF".split(/[\s\|]/)
=> ["y", "Y", "yes", "Yes", "YES", "n", "N", "no", "No", "NO", "", "true", "True", "TRUE", "false", "False", "FALSE", "", "on", "On", "ON", "off", "Off", "OFF"]

irb> yaml_1_1_bools.reject { |bool| bool.to_yaml.match?(/['"]/) }
=> ["Y", "N"]

I can make a PR if you like.