Update composer.json

The package name change to texy/texy:
https://github.com/dg/texy/commit/fc5fc99f40aca133f648f34a9b3fec3ad1302091

... this is no problem at all as long as you're using `packagist`. But `satis` cannot resolve `dg/texy` because it uses the package name from the `composer.json` (from https://github.com/dg/texy/blob/master/composer.json) which is `texy/texy`.