Change which rake task reports the version

We currently use "rake version -s" to produce the version number of the
project. However, if someone has rake installed but hasn't run rake
package:bootstrap, then rake will retun nothing, but still exit with a
0.

This changes which task we execute to the more specific rake
package:version; that does the exact same thing as rake version, but
will be a hard failure if the packaging shims haven't been put in place.
Merge pull request #495 from grimradical/rake-package-version

Change which rake task reports the version