Update to 2.8.0
    
    - API Changes
      * Add Modulemd.Module.get_translation() - Retrieve the translations
        associated with a Modulemd.Module
      * Add ModuleIndex.update_from_defaults_directory() - Import defaults from a
        directory of yaml documents, such as fedora-module-defaults, optionally
        providing a second path containing overrides.
    - Enhancements
      * Modulemd.ModuleIndex.update_from_file() now supports reading files
        compressed with gzip, bzip2 or xz. (Issue: #208)
      * Documentation updates
    - Bugfixes
      * Assorted minor issues discovered by static analysis tools.
    
    Signed-off-by: Stephen Gallagher <sgallagh@redhat.com>