Use Timestamp for timestamp on ir.cache

issue11290
review390181003
(grafted from 77582defbb9be076986d7b8b028d12d9f2812c4a)

--HG--
branch : 6.0

trytond/ir/cache.py

@@ -7,4 +7,4 @@ class Cache(ModelSQL):
     "Cache"
     __name__ = 'ir.cache'
     name = fields.Char('Name', required=True)
-    timestamp = fields.DateTime('Timestamp')
+    timestamp = fields.Timestamp("Timestamp")