(PUP-2781) Generate correct dependency expressions for new modules.

Since Puppet 3.6.0, puppet module generate has automatically created
a metadata.json file for users, with a sample dependency on
puppetlabs-stdlib to help demonstrate the syntax for expressing
dependencies. Unfortunately, the generated sample used an incorrect
key name (version_range vs version_requirement), which is less
useful and was introducing other problems with (e.g.) publishing to the
Forge.

This change repairs that mistake.