limit certain polyfill BC tests to 7.x
limit certain polyfill BC tests to 7.x

emeth-/heroku-buildpack-php,emeth-/heroku-buildpack-php,emeth-/heroku-buildpack-php,heroku/heroku-buildpack-php,heroku/heroku-buildpack-php,heroku/heroku-buildpack-php,emeth-/heroku-buildpack-php,heroku/heroku-buildpack-php