Work with RDoc 4

This updates hanna-nouveau to work with RDoc 4, based on a commit by Atsushi Nagase in his hanna-bootstrap fork (https://github.com/ngs/hanna-bootstrap/commit/f8f5968c14e0635546bc27ab835296bc6a3c063e).

Additionally, to make it easier since I know there is very little work on this project, I've also added a commit that bumps the version, updates the documentation to mention RDoc 4, and fixes the gemspec so that current rubygems will actually build a gem from it.

I've tested this with Sequel's documentation and it appears to work fine, though I didn't do a thorough review.  I'd like to get this merged and a new gem released.  If there are regressions with RDoc 4, I'll try to fix them, but the current situation of having to use RDoc 3 to use hanna-nouveau is a pain, especially on current versions of ruby that ship RDoc 4.