Update module build scaffolding.

Update the .fixtures.yml for proper YAML formatting.
Update the .gemfile to source gems from https://rubygems.org to fix a
Travis/bundler warning.
Add puppet-lint to the .gemfile to fix Travis.
Update .travis.yml to support updated Puppet versions.
Update Modulefile dependency to puppetlabs/stdlib >=2.2.0.
Standardize README.md header.
Add :validate and :lint to Rakefile.
Update puppetdoc boilerplate.