Download installer script from Github

I've been seeing intermittent issues where VIP Quickstart is having issues downloading the installer script from wp-cli.org. It seems to be fixed by downloading directly from Github instead. Additionally, we should probably re-download the installer each time. If there's an issue downloading it and a blank file gets created, it would be stuck with a blank installer forever unless you manually delete it.
Merge pull request #15 from joshbetz/patch-1

Download installer script from Github