We should build and test Qt on macOS 10.11.
As per http://code.qt.io/cgit/qt/qt5.git/tree/coin/platform_configs/default.txt?h=5.6#n17 this is the case on the Qt 5.6 branch. However all branches above (5.7, 5.8, 5.9, etc.) are not building and testing on 10.11. This is a regression towards 5.6 and as agreed in http://lists.qt-project.org/pipermail/releasing/2017-January/004436.html this is a release blocking issue for 5.9.