Added a Rake task to update just the agency list.
Added a Rake task to update just the agency list.

GSA/social-media-registry,ctacdev/social-media-registry,GSA/social-media-registry,GSA/social-media-registry,GSA/social-media-registry,ctacdev/social-media-registry,ctacdev/social-media-registry,ctacdev/social-media-registry