Guts/bundler: Restrict ourselves to ActiveRecord < 4.0

To retain Ruby 1.8.7 compatibility, we have to be pessimistic about
ActiveRecord. 4.x won't install at all.
Fix non-working links in deployment guide sidebar

For some reason, the {% iflink %} tags don't work with naked filenames; they
need ./ in front. This is definitely my fault, but I'm not going to get
elbow-deep in Jekyll extensions today.