Remove explicit version from composer.json

It's unnecessary and if it's not correctly maintained it breaks things.

For example here: https://github.com/steadlane/silverstripe-cloudflare/blob/1.3.0/composer.json

 * tag is 1.3.0
 * version is set to 1.2

This makes it very difficult to install the version.

Best fix is to remove this from composer.json and let composer infer the version based on tags and branches