*: encode canonical spec version in VERSION

The versioning of the spec should be decoupled from that of the golang schema
and tooling (e.g. `actool` and the ACE validator). This patch introduces a
`VERSION` file in the root of the repo to encapsulate the canonical spec
version. For now, the golang version is tied to this version (enforced through
the `test` script), but in future they may diverge (if for example the tooling
moves to its own repo).

This change also introduces a `+git` suffix to the version to reflect the real
state of things and avoid confusion with the actual 0.1.1 release.

For more context, see the discussion in https://github.com/appc/spec/pull/47
hrm, the other way to go would be a simple program that we `go run`. Looks OK to me though.