Use the yard 0.8.7.6 or greater

We can use the latest release of yard as it contains lsegal/yard#765

I wasn't too sure if I should explicitly require it to use the latest version or not
`gem 'yard' , '>= 0.8.7.6'` vs just `gem 'yard'`

If you guys have a preference, I can change it

Thanks a lot!