Double quote strings that contain single quotes only (retry)

[hsbt]

Original pull request: https://github.com/ruby/psych/pull/325

@knugie Your changes was different behavior with bundler behavior: https://github.com/bundler/bundler/blob/master/lib/bundler/yaml_serializer.rb#L8

There is no time to coordinate this specification for Ruby 2.5.0. I try to this next year(Ruby 2.6 and psych 3.1.0)