FileVersionInfo Cleanup

This addresses code review feedback from #274.

/cc @mellinoe and @stephentoub
Might want to add a comment on the choice of the constant here.  I think it's reasonable, but we should note why it was chosen.  FWIW, I just ran a little test where I looked at the length of ToString for all of the files in my c:\Windows\Microsoft.NET directory; the min was 332, the mean was 512 (with exactly 50% <= 512 in length), and the max was 802.