In light of the large number of issues upgrading OpenCV from 3.2 to 3.3, which was expected to not be an issue, we will not plan to update it again. As of Melodic we are planning to use the system installed version of opencv (with a few custom backports for older platforms) as the standard integration version. Discussion here: https://github.com/ros-infrastructure/rep/pull/160#issuecomment-375420063