Require gear 0.5.1

* requirements.txt: The latest release of the gear library fixes an
enqueuing/function registration order problem exposed by the recent
zuul merger fan-out implementation. Insist upon it.

Change-Id: Id2275477b12ce62fc485e5769029fe107a82b52d
diff --git a/requirements.txt b/requirements.txt
index 170b5152..92bb296 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -13,5 +13,5 @@
 extras
 statsd>=1.0.0,<3.0
 voluptuous>=0.7
-gear>=0.4.0,<1.0.0
+gear>=0.5.1,<1.0.0
 apscheduler>=2.1.1,<3.0