Fail build if coverage is less than 100%.

For #461.

This will fail initially; I'm just getting the build going.
We'll have to doc everything before we can merge this.

We'll also want to extract this into rspec-dev
(and propagate it to all repos) once we're happy with
this build change.
Always install yard as part of builds.

It's needed for our new check_documentation_coverage step.
Use `bin/yard`.
Fix invalid YARD tag.
Get docs up to 100% coverage.
Merge pull request #482 from rspec/100-percent-doc-coverage

Fail build if coverage is less than 100%.