fix race condition in record-fact-existence!
fix race condition in record-fact-existence!

Previously, running several facts in parallel would occasionally cause
'AssertionError Assert failed: (not (neg? index-to-exclude))' in
midje.data.compendium.Compendium/vector-remove.

Turns out that was due to a race condition in
midje.data.compendium/record-fact-existence!.

This commit fixes it.

aeriksson/Midje,bens/Midje,marick/Midje