Add pre packaging rake tasks

Prior to this commit, nothing existed for packaging to be able to pull
down and vendor the necessary gems from the Gemfile. This commit adds
two specific rake tasks which can be exercised during the packaging
pipeline: `packaging:pretar` and `packaging:prebuild`.

`packaging:pretar` will pull down the dependencies specified by
`Gemfile` and cache them (in an unbuilt form in `vendor/cache`. Then
when the package source tar is created, the dependency gems will be part
of that tarball. Gem dependencies of `packaging:pretar` are `rake` and
`bundler`.

`packaging:prebuid` will take the vendored gems from the tarball and
build and install them into `vendor/bundle`.  Any commands prefaced with
`bundle exec` will use this vendored gemset bundle. Gem dependencies of
`packaging:prebuild` are `rake` and `bundler`.