gdcm: 2.6.8
gdcm: 2.6.8

Closes #5586.

Signed-off-by: Michka Popoff <7b0496f66f66ee22a38826c310c38b415671b832@gmail.com>

quantumsteve/homebrew-science,heuermh/homebrew-science,dpo/homebrew-science,andrewjpage/homebrew-science,pditommaso/homebrew-science,philippbayer/homebrew-science