Removed version property from composer.json

This will enable versions for composer/packagist to be managed soley by
tags and no longer require the version property to be updated manaually.
This subsequently resolves future issues with tagged releases from not
showing up due to forgoted version updates to the `composer.json`
Merge pull request #10 from zircote/feature/remove-composer.json-version-property

Removed version property from composer.json