*: move spec to new home

The appc spec has moved! Its new home is at https://github.com/appc/spec

This removes the spec (since it has already been migrated), and adds 
placeholders linking to the new repo. It also updates rocket to reference the
spec repo _directly_ (i.e. NOT godepped). This means that people will need to
maintain the spec in their gopath for now when building rocket.