Add package:vendor_gems tasks

Prior to this commit, the packaging repo had no way of vendoring gems to
be included in a source tarball. This commit add such a task so that
prior to running `rake package:tar` a package builder can run `rake
package:vendor_gems` and the necessary gems for the project will be
pulled down and stored in `vendor/cache`.

NOTE: Later steps in the build process will be required for making use
of these cached gems. Those later steps might look something like:
```
bundle install --local --deployment
rm -rf vendor/cache
```