Fix post revision action link by getting the adjacent revision

Taking another stab at #585, this time using a workaround that finds the adjacent revision ID.

Unfortunately, the `revision_id` being saved as record meta is still incorrect. But this PR does fix the action link problem to arrive to the correct revision for that record.

![screen shot 2014-11-13 at 9 27 30 pm](https://cloud.githubusercontent.com/assets/522158/5040894/e7cf2fec-6b7b-11e4-927b-4402758f1013.png)

@shadyvb please review.
@fjarrett `get_adjacent_post_revision` already does that check internally. Also i think we should first check if `$revision_id` is empty or not, so as not to make the extra function call if it was empty.