Env var 'scala_ref' decides scala/scala gitref to build.
Centralize git-refs for repositories

Since dbuild's long-term goal is to require only a list of github url's
to build, lets already encode that in our configs, so that the header
tells you what's being built, and everything below that is hackery
to deal with imperfections in the various builds/dbuild.

I verified the following PRs were merged, so I changed the refs to
the corresponding branches:
```
etorreborre/specs2.git#master -> etorreborre/specs2.git#pull/238/head
milessabin/shapeless.git#pull/80/head -> milessabin/shapeless.git#scala-2.11.x
```

I double-checked the patch, which is the result of the following regex hack:

- create the vars section: `s|([\w-]+)/([\w-]+)(#.*\")?|$2-ref: "$1/$2$3"|`
- replace uri's by standardized `"https://github.com/"${vars.$2-ref}`:
`s|/([\w-]+)/([\w-]+)(#.*\")?|/"{vars.$2-ref}|g`
Cleanup. Centralize SBT version overrides.
Moved comments describing forks to vars, sorted.
Override repos, needed to add spray.io repo.