PuppetDB feature release 1.4 indexing changes

(Hi, I need a pre-review of this one, as we're prepping for a new PuppetDB feature release. We'll give you a shout when we're ready to merge ...)

This includes indexing changes for the new feature release for PuppetDB 1.4:

* A new version index page has been included, and referenced from the main
  index page.
* The source retrieval config now references the new 1.4.x branch.

Signed-off-by: Ken Barber <ken@bob.sh>