Log Message:
lsix: update to 1.9.1.

1.9.1

Turns out ImageMagick 7 has deprecated using magick convert as
well, which is silly. Our solution: alias both convert and montage
to do the right thing for IM7. This is uglier than the previous
solution, but that's okay if it works.

1.9

Use IM7's 'magick' binary if it exists. Closes #73. Closes #78.