Use latest platform versions by default

Remove all the default.json files and thus default to the newest (well,
based on filename sorting) version if `:version` option is not
specified. If the filename sorting is not valid with some platform in
the future, a default.json can be then added for that platform.

The whole feature is undocumented and users are encouraged to always
declare the specific version anyway.
Remove default.json searching logic
Merge pull request #25 from tmatilai/use_latest_versions_by_default

Use latest platform versions by default