mip: github branch is incorrectly used for secondary github: sources in package.json

[ned-pcs]

mip version 0.2.0

When you specify a version to mip.install() to reference a particular GitHub branch, this branch is used for every github: source in the package.json, even those coming from a different repository.

The best solution to this is probably:

• provide a way (perhaps an optional third field) for a URL to specify a different branch
• default the branch to HEAD for secondary github: urls if they don't provide an explicit branch

In my PR #676 I have done the second part of this (default to HEAD) but not the first part yet.