Add missing return to `fold`.

Fixes #88.
Bring back output formatting improvements.

I apparently pushed these changes to each repo but
forgot to commit them to rspec-dev so we lost them
on the last change.
Check for warnings when generating YARD docs as well.
Don’t fold clones.

It’s already within a travis fold and nested folds don’t work.
Fail on yard errors, too.
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.
Merge pull request #89 from rspec/fix-fold

Add missing return to `fold`.