Only enforce YARD documentation coverage on MRI 2+.

- We now fail the build if generating the docs emits
  any `[error]` lines, and all other versions are
  getting an error notification about redcarpet
  not being available.
- Ruby 1.8 and 1.9 are getting some other odd errors
  from YARD that ruby 2.0 doesn't get.

Ultimately, we only care to ensure that generating the
docs fully works when done with a current version of
ruby (since that's what we use when generating them...)
so it makes sense to skip older versions.