allow for version branches, eg: 5.0.x-dev

[Jimbolino]

I'm not sure how packagist internally works, but an example can be seen here:

https://packagist.org/p/laravel/framework.json
https://packagist.org/packages/laravel/framework
https://github.com/laravel/framework/branches

As you can see there is a "5.0.x-dev" that refers to the "5.0" branch on github.

Based on what is written here:
https://getcomposer.org/doc/02-libraries.md#branches

I would suggest that any branch that looks like a version, should be listed as 5.0.x-dev

[lemoinem]

Follow up on #14

If you were able to provide a PR implementing this change, I'd be glad to merge it.