PuppetDB feature release 1.4 indexing changes

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>
Merge pull request #201 from kbarber/puppetdb_14_docs

PuppetDB feature release 1.4 indexing changes