minor fix: heroku-18 elif was wrong
minor fix: heroku-18 elif was wrong

made no difference in practices, since the libraries are already there on heroku-18; that whole list of needed packages is more declarative than anything else

emeth-/heroku-buildpack-php,heroku/heroku-buildpack-php,alzheic/heroku-buildpack-aop,emeth-/heroku-buildpack-php,emeth-/heroku-buildpack-php,heroku/heroku-buildpack-php,emeth-/heroku-buildpack-php,heroku/heroku-buildpack-php,alzheic/heroku-buildpack-aop,alzheic/heroku-buildpack-aop,heroku/heroku-buildpack-php