Until QGIS on the Ubuntugis repos gets built with the updated GDAL packages the easiest workaround I've found to get around this dependency issue is to create a fake virtual gdal-abi-2-1-2 package using equivs-build per the instructions here: Cannot install QGIS 2.18 from repository on Debian Stretch; package "gdal-abi-2-1-1" not installableCannot install QGIS 2.18 from repository on Debian Stretch; package "gdal-abi-2-1-1" not installable
QGIS should then update to 2.18.5 and run normally.