Version fix

As I noticed in https://github.com/JuliaLang/julia/commit/984871d35f9856702fb4a08812d44cfbbccb2f30#commitcomment-4651002, the `-prerelease` flag only gets appended to the version if git info is availible while building. This PR removes the build-time prerelease flag, and expects the flag to be present in `/VERSION` instead.

I also changed the type of `BUILD_INFO.fork_master_timestamp`, to Float64 because everybody should stop using 32 bit integers for timestamps. Even though it does not matter in this case.