Revert "Permit YAML blocks to end with three dots"

This reverts commit 52ac2b3850e25c783ec31c46eda948cffd861452, introduced in
v2.0.0 by @lmullen. It has been reverted because it adds complication to
the process of writing Jekyll sites ("Do I use dots or dashes here??") and
because the sole purpose was to bend to the will of pandoc, which could
just as easily prefer dashes to dots.

Ref: #2110, 3134

- [x] Revert the commit
- [ ] Make it backwards-compatible such that it doesn't break sites that are using it.