Avoid raw github POST for making API request

#212 adds a new raw POST request to request generation of the release notes. This is because the library we're using to interface with the API doesn't have an abstraction for that yet - but I have created a PR in that repository to create one: https://github.com/KnpLabs/php-github-api/pull/1074

Once that is merged and released, we should use the abstraction.