Rename `module-data` to `module_data`

This came up in rodjek/librarian-puppet#242. A `-` is not a valid
character in module names according to
http://docs.puppetlabs.com/puppet/latest/reference/modules_fundamentals.html#allowed-module-names
In the Modulefile for ripienaar/puppet-module-data, the name actually
contains an underscore.

Thanks @hanjianwei for noticing this and tracking down the naming rules
Merge pull request #640 from boxen/fix-module-data-name

Rename `module-data` to `module_data`