Log Message:
Update ruby-rdoc to 4.2.1.

4.2.1 / 2015-12-22

Bug fixes

* Fixed infinite loop with CR #339 by @nobu
* Allow rdoc run with disable-gems #340 by @luizluca
* Don't store full path in GZipped js files #341 by @voxik
* Fix relative path names for drive letters #367 by @nobu
* Fix for valid syntax `class C end` parsing #368 by @nobu