fix copy-packages in zsh (without breaking sh and bash)

I created this patch to fix #299. 

The problem is with zsh not splitting words in variables before passing the value into commands ([details](http://zsh.sourceforge.net/FAQ/zshfaq03.html#31)).

This patch converts $INSTALLS into an array, and passes the array into `npm install`. It is a non-obtrusive fix. The patch in #223 uses `eval`, which can be dangerous. This doesn't use `eval`, or `setopt shwordsplit`, or any of that jazz, I've tried to keep it as vanilla as possible, without breaking sh/bash, so that you can accept this pull request into the main repo.

Thanks,
Marcin