Fix broken Composer version constraints

The last commit added invalid version constraints to your composer.json file (see: https://getcomposer.org/doc/faqs/why-are-version-constraints-combining-comparisons-and-wildcards-a-bad-idea.md). This means that anybody who has been using your package won't be able to run `composer update`.

This change redoes the version constraints so that it is sensible and works as expected.