QGIS removed after updating to 2.18.5 on ubuntu

This is indeed a bug in the QGIS installer, and already has a ticket: http://hub.qgis.org/issues/16383

For the moment, only QGIS-LTR is running on trusty. You have to remove and purge the packages of QGIS 2.18 to avoid versioning conflicts: https://askubuntu.com/questions/187888/what-is-the-correct-way-to-completely-remove-an-application

Or consider upgrading to Linux Mint 18.1, which is based on xenial. The missing packages are available for that.

Installing python-future with pip does not help, since the package manager does not see that.

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UPDATE

The ticket has been closed as fixed. Everything should be back to normal now.