backwards compatible support for 0.9.0

Not sure why this didn't automigrate... all I did was touch up the package.js file.  Did so in a way that will still work on pre-0.9.0 too.
Uhhh :(  Seems there are two hackpads, both modified yesterday:

https://meteor.hackpad.com/Unipackage-tvas8pXYMOW#:h=Publishing-a-New-Package
https://meteor.hackpad.com/Getting-Started-With-Unipackage-IOckQBQgXaQ#:h=Anatomy-of-a-Package

The first says "git" and the second says "githubUrl".  Personally I prefer the former.  Looking through Meteor source now to figure out what's going on here :>