Made -prerelease flag in VERSION independent of git access.

Also removed prerelease boolean in BUILD_INFO. Users should use
VERSION.prerelease == "" to test for that instead.
Changed type of BUILD_INFO.fork_master_timestamp from Int to Float64
Merge pull request #4863 from ivarne/version_fix

Move `-prerelease` version modifier into `/VERSION`, ensuring that it is kept in builds lacking git information.