Fiji/fiji:latest docker image updated

A heads up for anyone who has used the https://hub.docker.com/r/fiji/fiji Docker image in the past that using the latest tag will now produce an OpenJDK8-based container. If you are looking for the previous image, use the fiji-openjdk-7-legacy tag.

For details, see the discussion in https://github.com/fiji/dockerfiles/pull/11

Testing and feedback welcome.
~Josh