Pin yard version to v0.9.28 to avoid ArgumentError

It seems as if the recent v0.9.29 release of yard broke my custom
template code. I plan to report this to yard and to come up with a fix,
but in the meantime this fixes the CircleCI build. Without this change
we were seeing the following error when attempting to generate the docs:

    ArgumentError: No such template for default/fulldoc/html

Gemfile

@@ -26,5 +26,5 @@ if RUBY_VERSION >= '2.2.0'
 end
 if ENV['MOCHA_GENERATE_DOCS']
   gem 'redcarpet'
-  gem 'yard'
+  gem 'yard', '<= 0.9.28'
 end