Update download.html for 4.0

This commit changes the version variable to reflect the latest 4.0
release and modifies the url to account for the shift from geppetto/
to geppetto/4.x.
s/cloudsmith/puppetlabs

This commit largely replaces references to Cloudsmith in copy and
urls to Puppet Labs. A few minor typos or copy corrections are
included.
Merge pull request #2 from rcoleman/gh-pages

update puppetlabs.github.io/geppetto for 4.0 and PL transition