*: 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.
Merge pull request #257 from jonboulle/move_appc

*: move spec to new home