Fix YUIDoc generation.

The builds were not publishing to S3 because `ember yuidoc` command did
not exist after refactoring the Brocfile into its own repo.  This brings
back the addon (and therefore the command) so that we can properly
generate the data.json output.
Merge pull request #10013 from rwjblue/add-ember-cli-yuidoc

Fix YUIDoc generation.