Add require-approvals to EventFilter representation

We can request additional approval to events since c053d026
https://review.openstack.org/#/c/68516/ .  The EventFilter.__repr__ was
not displaying them though.

Change-Id: I966699e8d2daba6bd2c3bb8d57ef6d7a0e3177f2
diff --git a/zuul/model.py b/zuul/model.py
index 2f4110f..d017744 100644
--- a/zuul/model.py
+++ b/zuul/model.py
@@ -1009,6 +1009,9 @@
         if self.event_approvals:
             ret += ' event_approvals: %s' % ', '.join(
                 ['%s:%s' % a for a in self.event_approvals.items()])
+        if self.require_approvals:
+            ret += ' require_approvals: %s' % ', '.join(
+                ['%s' % a for a in self.require_approvals])
         if self._comment_filters:
             ret += ' comment_filters: %s' % ', '.join(self._comment_filters)
         if self._email_filters: