pin package versions for python-pip to bionic-security
pin package versions for python-pip to bionic-security

so that we do not get an update of libssl to 1.1.1, since the stack is still on 1.1.0

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