require minimum postgres version to use the date_range

.travis.yml

@@ -8,6 +8,7 @@ sudo: false
 cache: pip
 
 addons:
+  postgresql: "9.2" # minimal postgresql version for the daterange method
   apt:
     packages:
       - expect-dev  # provides unbuffer utility

stock_demand_estimate/models/date_range.py

@@ -17,4 +17,4 @@ class DateRange(models.Model):
                             fields.Date.from_string(rec.date_end)).days + 1
 
     days = fields.Float(string="Days between dates",
-                        compute='_compute_days', store=True, readonly=True)
+                        compute='_compute_days', readonly=True)