FileVersionInfo Cleanup

This addresses code review feedback from #274.

/cc @mellinoe and @stephentoub
I was originally going to go with `string.Format(CultureInfo.InvariantCulture, "{0:X8}", value)`, as you suggested, but `value.ToString("X8", CultureInfo.InvariantCulture)` is even faster (less allocations, and I'm guessing faster format parsing).