PDB-370 Prepare 1.6.0 PuppetDB documentation for final release

This does two things:

* Removes the warning that this is a pre-release version
* Changes the index so that 1.6 is now marked as 'stable'

Signed-off-by: Ken Barber <ken@bob.sh>
Merge pull request #276 from kbarber/ticket/master/PDB-370-make-puppetdb-160-a-final-release

PDB-370 Prepare 1.6.0 PuppetDB documentation for final release