changed appdata <licence> tag to the new <metadata_license> tag

changed the <licence> tag to the new <metadata_license> tag to reflect a change in the Appdata Spec that deprecated the <licence> tag, and replaced it with the <metadata_license> tag to remove ambiguity between the project license and the metadata license

See:
http://people.freedesktop.org/~hughsient/appdata/#metadata_license