Switch to haml 4+ and sass instead of forcing haml 3.0.25
Switch to haml 4+ and sass instead of forcing haml 3.0.25

Tests have not shown a significant change in output.

Also, don't force RDoc 4, since there is no known incompatibility
with RDoc 5.

rdoc/hanna-nouveau,rdoc/hanna-nouveau,rdoc/hanna-nouveau