Use npm to pull in the appropriate version of bootbox instead of including a static file.

Meteor 0.6.0+ now includes support for npm. We can pull in bootbox directly using npm instead of including a static file, and it becomes much easier to bump versions without making the repo include all copies of bootbox, etc.

I thought I would do this instead of creating a new package on atmosphere, but if you disagree, no problem - I'll just make a separate release on atmosphere.