Removing hyperlink to ImageJ.
Removing hyperlink to ImageJ.

The NIH URL was unstable resulting in failed tests. This is
intermitent, so I am removing the link.

InsightSoftwareConsortium/SimpleITK-Notebooks,zivy/SimpleITK-Notebooks,InsightSoftwareConsortium/SimpleITK-Notebooks,zivy/SimpleITK-Notebooks,InsightSoftwareConsortium/SimpleITK-Notebooks,zivy/SimpleITK-Notebooks