Sphinx: get Julia version from ../VERSION

* Falls back to `X.Y`, `X.Y.Z-unknown`, or `vX.Y` if `../VERSION` does not exist
* `release` is now equal to VERSION (currently `0.3.0-prerelease`), rather than `0.3.0-dev`

Regarding this last point, is there anything that depends on the `0.3.0-dev` format?

c.f.: https://github.com/JuliaLang/julia/commit/a8dbe6bd5219285681870a530d4f449f89b7b6c8#commitcomment-5609755

cc: @JeffBezanson, @ivarne, @staticfloat, @nolta