This change doesn't really affect the installation speed, but it's good
to be consistent with the version used to build both nextstrain.org and
auspice.us.

This prevents running Auspice's prepare script, which is not necessary
for installation without usage in the same environment.

This prevents installation of devDependencies in the current project.
There are none at the moment for both nextstrain.org and auspice.us, but
if that ever changes in the future, we would want to ignore them in this
step.