update PHP base download URL and add 7.4 test download URL
update PHP base download URL and add 7.4 test download URL

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