COMP: use mirror of NiftyReg repo. #116.
COMP: use mirror of NiftyReg repo. #116.

The certificates of https://cmiclab.cs.ucl.ac.uk/mmodat/niftyreg have
expired, making the download by our Jenkins CI fail.

SuperElastix/SuperElastix,kaspermarstal/SuperElastix,SuperElastix/SuperElastix,kaspermarstal/SuperElastix,kaspermarstal/SuperElastix,SuperElastix/SuperElastix,SuperElastix/SuperElastix