Add “update” argument to bootstrap script.

- It will update the pods (i.e. run `pod update` instead of `pod install`).

Signed-off-by: Aron Cedercrantz <aron@cedercrantz.se>
Pod update did this when I tested the new bootstrap param.

Signed-off-by: Aron Cedercrantz <aron@cedercrantz.se>