Change all occurences of `exercism/representer` to `exercism/php-representer`

[homersimpsons]

It should match the tag used in https://github.com/exercism/php-representer/blob/main/bin/run-tests-in-docker.sh#L16, so this is fine. You could change all occurences of exercism/representer to exercism/php-representer in a later PR.

Originally posted by @ErikSchierboom in #179 (comment)