Update OCMock to version 3.7.1

This gets us to latest and fixes a warning about a missing cast.

Related patch: flutter/buildroot#401

Author: cbracken

DEPS

@@ -68,7 +68,7 @@ vars = {
   'dart_usage_tag': '3.4.0',
   'dart_watcher_rev': 'fc3c9aae5d31d707b3013b42634dde8d8a1161b4',
 
-  'ocmock_tag': 'v3.4.3',
+  'ocmock_tag': 'v3.7.1',
 
   # Build bot tooling for iOS
   'ios_tools_revision': '69b7c1b160e7107a6a98d948363772dc9caea46f',