Rename apache::mod::default to apache::default_mods

Similar to `apache::mod::dev` being moved to `apache::dev`,
`apache::mod::default` is not a module itself, just a collection of
httpd modules and so shouldn't exist in the `apache::mod::*` namespace.
Merge pull request #8 from hunner/rename_default_mods

Rename apache::mod::default to apache::default_mods