update yaml to v0.9.0

[braydonk]

Updates yaml library to v0.9.0. This includes braydonk/yaml#21

Fixes #236 and #147

[braydonk]

Note to future self: One change caught by integration tests, but I don't think it's a big deal. In the past, The head comment of the first element when include_document_start is not set would have a newline due to the old way the yaml library handled document start head comments (putting in a break indiscriminately 🤷)

Now this will no longer happen. However, if a user actually wants the initial forced newline after the top comment in a document, they can turn on retain_line_breaks and put that newline in and it will still work the same as always.