Allow API key to be set by yaml config.

This change means that the Raygun API key can be set in two different ways:

1. Firstly, it checks your YAML configuration for the api_key value to be set on RaygunLogWriter.
2. If it can't find that, it will look for the SS_RAYGUN_APP_KEY define, which can be set in your _ss_environment.php (this was the previous default behaviour).
    1. Note that it will only search in SS_RAYGUN_APP_KEY if it can't find the YAML configuration value first.