Change header ordering from a MUST to SHOULD clause

We had this as a 'MUST' because the directory authorities need to agree upon an ordering in their votes to derive at a signed consensus. However, this requirement is turning out to be very error prone... https://trac.torproject.org/projects/tor/ticket/21059 Got the ordering issues sorted out... I think. However, this has bitten us three times in a row and clearly will bite us more in the future so loosening these requirements a little.

Change header ordering from a MUST to SHOULD clause
578477a6 · Damian Johnson · 3c91ffac · 578477a6
Commit 578477a6 authored Jan 2, 2017 by Damian Johnson
--- a/dir-spec.txt
+++ b/dir-spec.txt
@@ -1526,7 +1526,7 @@
   Some items appear only in votes, and some items appear only in
   consensuses.  Unless specified, items occur in both.

-   The preamble contains the following items.  They MUST occur in the
+   The preamble contains the following items.  They SHOULD occur in the
   order given here:

    "network-status-version" SP version NL