Refactored how we generate the advisory blog posts.

* git clone/pull ruby-advisory-db instead of using submodules.
* Put all of the advisory metadata into the YAML front matter.
* Render the advisory posts using the advisory layout.
* Place advisory blog posts in /advisories/_posts/ instead of /_posts.
Added an advisories:commit task.
Allow empty commits.
Fixed how we get the sha1 of _advisories/.
Updated advisory posts against rubysec/ruby-advisory-db@0120fe7
Remove the default task, since people might accidentally run it.
Updated the README.