PHP 7.3 Support - Symfony YAML parser - APCU Cache adapter

This release fixes some notices/warnings that were thrown by PHP 7.3.

There's now an APCU cache adapter that can be used, which may eliminate the need for the apcu_bc extension (thanks @rossmotley!).

YAML parsing has also been changed from the bundled sfYaml to Symfony's YAML parser. Due to the differences in Symfony\Yaml and sfYaml we're releasing this as a new major version as the Symfony\Yaml component will throw exceptions in cases of improper YAML syntax, where the sfYaml parser would not (more details in #43). Otherwise there have been no BC breaks vs. the 2.x version.