Add Package-Requires header for ELPA installations

This commit also fixes the header format so that package repositories such as Marmalade can extract the commentary text.

P.S. I'd like to add a recipe for troncle to [MELPA](http://melpa.milkbox.net/), but it would need to be updated for the recent `nrepl` -> `cider` rename first. Please ping me when/if that gets done in case I don't notice. :-)