use arithmetics in curl_retry_on_18 while loop
use arithmetics in curl_retry_on_18 while loop

((attempts++)) evaluates to 1 on first call, causing an error with set -o errtrace

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