Just a quick note, that this doesn't work on Ubuntu 16.04, as this version uses systemd, and after some small troubleshooting @lmori came up with the following solution for the problem:
@borising ok, now I understand why it's not working: Ubuntu 16.04 LTS uses systemd. To add the environment variables to MineMeld you should do `sudo systemctl edit minemeld.service` and then paste something like
luigimori [10:29 PM]
added this Plain Text snippet: Untitled
[Service]
Environment=HTTPS_PROXY=<...>
Environment=HTTP_PROXY=<...>
luigimori [10:29 PM]
`sudo systemctl daemon-reload`
`sudo systemctl restart minemeld`
And after I did this, it worked as expected.