http://packages.magento.com is now https://repo.magento.com

[treii28]

I was about ready to post another comment as composer/git complained about pulling from a non-https source, but when I set security off in the config, it turned out to be not-found. It looks like the repo is now on an https connection at a different fqdn. I modified the composer.json and it seemed to work to do a composer update

[treii28]

it also appears that it's installing a number of php7.1 extensions in conjunction with a php7.0 install and composer gets confusing not finding extensions such as 'dom' (part of php-xml), 'mcrypt', 'zip', 'mbstring' and 'intl'. Doing an apt-get install of the php7.0 versions of these packages seemed to make composer update happy.

e.g.
sudo apt-get update && sudo apt-get install php7.0-xml php7.0-mcrypt php7.0-zip php7.0-mbstring php7.0-intl