Raise error when more than one asset manifest file is detected

If a user is upgrading from Rails 3 to 4, the assets:precompile task will ignore the manifest.yml
that was generated in Rails 3 and generate its own JSON manifest. It's impossible for us to know
which manifest the user's current code requires, so it's important that we only have one manifest
in the assets directory at a time. When we detect two manifests, we raise an error pointing the
user towards a wiki entry explaining the problem and instructing them to remove the Rails 3.x manifest.
Properly deal with the asset manifest files when a user is rolling back from Rails 4 to 3.

If a user rolls back an application that was upgraded to Rails 4 to their Rails 3 code,
we need to remove their JSON Rails 4 asset manifest file and restore their YAML Rails 3
manifest instead of blindly restoring their manifest to whatever their current manifest
is named, as we do in all other cases.
Merge pull request #461 from jimryan/handle-assets-manifest-during-rails-upgrade

Handle assets manifest during rails upgrade.