Rename block_ruler to block_thematic_break (fix)

It was silently renamed in this megacommit:
https://github.com/asciidoctor/asciidoctor/commit/d98c8e9cd5e5793fdab6da7a0a6f37eb8349878b
Merge pull request #103 from jirutka/patch-15

Rename block_ruler to block_thematic_break (fix)