libyaml 0.2.1 breaks YAML builder

[straight-shoota]

The new release libyaml 0.2.1 seems to change some behaviour which breaks a few of our yaml specs.
An example can be seen on any of the last spec runs on CircleCI OSX since the homebrew formula was updated a few hours ago (e.g. https://circleci.com/gh/crystal-lang/crystal/8409).

It seems like libyaml has removed writing the document end marker (...) and is more consistent now. It was already only added in a few places. So I guess the only thing to do here is to remove the expected \n... from the affected specs.

I'm not sure what exactly causes this as I didn't find any information about this in the libyaml release announcement or the commit history. But I have checked that these specs pass with libyaml 0.1.6 but fail with 0.2.1 on linux as well.
I didn't check 0.1.7 but that had been working in the homebrew formula until recently.

[bcardiff]

Thanks for investigating. Due to the nature of the change I think that the best thing is to change the assert_built in yaml specs to allow optionally the ...\n suffix.

[asterite]

I can tackle this

[straight-shoota]

This is caused by yaml/libyaml#18 which removes the erroneously inserted document end marker after a scalar in root context.