Update ruby-yard to 0.9.8.

# 0.9.8 - January 13th, 2017

- Fixed installed gems not being correctly found in `yard server` and by plugins.
- Fixed tokenization of `%w(...)` array syntax.

# 0.9.7 - January 9th, 2017

- Fix resolution of absolute object paths with ambiguous names. (#1029)

# 0.9.6 - January 7th, 2017

- Removed official support for Ruby 1.x (1.8/1.9). YARD can still be installed
  in these versions, but support is not guaranteed. Simple bug fixes may still
  be considered via pull request only. Issues without code will be automatically
  closed.
- Added {YARD::Tags::Tag#explain_types} returning a plain English summary
  of the type specification of a given tag. Also adds {YARD::Tags::TypesExplainer}
  as an implementation class for the method.
- Added support for automatic linking of constants and method calls of
  Ruby syntax highlighted source code in generated HTML. Also adds the
  {YARD::Parser::Ruby::TokenResolver} implementation class to iterate over
  tokenized code with extra resolved object information.
- Added support for compound constant assignments (`A::B::C = true`).
- Added `LibraryVersion#yardoc_file_for_SOURCE` callback method for sources with
  a pre-determined yardoc file location. Implement this method instead of
  manually setting `library.yardoc_file = ...` in your load method (you can
  still assign the attribute manually).
- Use RubyGems 2.x+ API to query gems when available instead of using backport.


(taca)