use explicit jekyll command over rake file

For some odd reason, the rake task we previously had would cause me to get 403 Forbidden from Webrick. Not sure if that was a permissions thing on the `Rakefile` or the rake script not having `bundle exec jekyll --server --auto` or what, seemed to be hit or miss. After a few times of getting a 403, i'd refresh hard enough and it'd finally go through to the page.


I personally don't use any rake commands often at all. I'm almost more in favor of only having one abstracted command to start something instead of having a `rake server` and `script/server` that do the same thing.