Pin PuppetDB 1.5.x docs to a tag

In prepration to remove the 1.5.x branch, we need to ensure our docs are
pointing a tag instead.

Signed-off-by: Ken Barber <ken@bob.sh>
Merge pull request #318 from kbarber/maint/master/pin-pdb-15x-docs-to-tag

Pin PuppetDB 1.5.x docs to a tag