fixed composer.json version

The version in `composer.json` was not in sync with the version declared with `Raven_Client::VERSION` (0.6.0 vs 0.6.1).

Anyway, one of the best practice when using Composer is to avoid setting a version in the `composer.json` file, and add a `branch-alias` instead. That way, Packagist can automatically create a dev version for master and automatically get the real version from the tags.