This commit changes the source and documentation to reference this project as
`puppet-strings` rather than `puppetlabs-strings`.
This makes the source and project match the gem name.
Prior to this commit, the gemspec for strings relied on the module
metadata.json. Since we would ultimately like to stop releasing
strings as a module entirely, update the gemspec not to rely on the
module metadata so it is not broken when we ultimately remove it.
Peter Huene