Aaargh... update TrakEM2 to a temporary fork

TrakEM2 references Fiji components with ${imagej.version} but that
no longer works. So we have to move the .gitmodules pointer to our
fork until the pull request gets accepted.

In the same run, we also have to update mpicbg because TrakEM2 no
longer compiles with the old version.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>