What happened to /etc/apt/apt.conf?
The /etc/apt/apt.conf file is still read if it exists. The apt configuration provided by packages is installed as separate files in /etc/apt/apt.conf.d/ so that it can be added and updated independent of any local changes you make in the main configuration file.
So if you have local configuration changes, just create the file.
Just add the following to /etc/apt/apt.conf and the problem is solved: sudo vi /etc/apt/apt.conf then add this to the first line:
Acquire::http::Proxy "http://proxy_address:8080/";