Detect client version automatically

[OndraM]

We should replace Matej::VERSION constant, which needs to be updated each release, with https://github.com/Ocramius/PackageVersions , which allows to read the version based on actual package version installed via Composer.

Current way we update the constant manually each release also causes BC break false positive each time the constant value is changed (as it is considered BC break by https://github.com/Roave/BackwardCompatibilityCheck )

However, the PackageVersions requires PHP 7 ¯\_(ツ)_/¯, so we are blocked until we need to support PHP-5 version of Matej client.