convert adheres to policy plugin into proper gem

* vendoring gems to allow the gems that we own to reference a locked
* down version in their gemspecs. Specifically, with the old version of
* ActiveSupport that we were getting from the LTS Rails repo, there is
* no way to tell bundler to install a gem that is a subdirectory of a
* git repo.

Change-Id: I32d9d0fae15da1fcf27f11e5cd8fca26af18d383
Signed-off-by: David Julia <djulia@pivotallabs.com>
Reviewed-on: https://gerrit.instructure.com/29438
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Stephan Hagemann <stephan@pivotallabs.com>
Product-Review: Stephan Hagemann <stephan@pivotallabs.com>
QA-Review: Stephan Hagemann <stephan@pivotallabs.com>