Rubypants: avoid to "educate_dashes" HTML comments close tags.

I'm unsure of the fix, but here are the step to trigger the bug:

```
% git clone https://github.com/imathis/octopress.git && cd octopress
% rake install
% curl https://gist.githubusercontent.com/kAworu/0a06ac86fbd819714d52/raw/975aeffdd67b8b932d550e46fe00877ea22f3caf/head.patch | patch -p0
% rake preview # oups.
```