Switch to GitHub Flavored Markdown dialect

[apjanke]

This is a prospective PR for switching to GFM, to fix an issue with fenced code block formatting brought up by an email submitter.

Fixes #57.

[apjanke]

Okay, here's something: maybe this should go into an 0.9.x patch release instead of an 0.10.0 minor release, for distribution reasons: Debian and the dev Ubuntu are on Ronn-ng 0.9.x, so they might pull in new 0.9.x versions, but probably wouldn't upgrade to a new minor version until the next distro releases. And getting fenced code blocks fixed is an important bug fix.

[apjanke]

I'm currently leaning towards making this fix an 0.9.2 release rather than 0.10.0; having fenced code blocks be broken is a bad enough regression that this should count as just a bugfix, and I think it'll be back-compatible.

[apjanke]

Another Ronn-NG user has reported #56, which looks to me like this same problem with fenced code block support being broken.

[apjanke]

Okay, the remaining Travis failures are just gem packaging failures. Don't know what's up with them; maybe it's a Ruby version dependency? Anyway, I think they're fine, since it's working locally for me, and at least one of them (Ruby 2.7) is passing on Travis.

Merging.