check for master package.json if no tag version

[jrburke]

Right now libs that want to upgrade to volo in their package.json it will not take affect until the first tagged release. It would be good if they could just commit to their master branch, and have volo check that version before going to the shim repo.

Ideally #37 is fixed too so that the correct master branch is chosen for the project. But chosing "master" for now would work.