Use new option to display a change link instead of previous hack (Django<1.8)

Django>=1.8 has an option (show_change_link) to display a change link, use it instead of the previous solution.

coin/members/admin.py

@@ -37,16 +37,9 @@ class OfferSubscriptionInline(admin.TabularInline):
     extra = 0
     exclude = ('comments',)
     readonly_fields = ('get_subscription_reference', 'subscription_date', 'resign_date',
-                       'commitment', 'offer', 'show_change_link')
-
-    # FIXME: Workaround en attendant la migration vers Django >=1.8
-    # À remplacer par InlineModelAdmin.show_change_link = True
-    def show_change_link(self, obj=None):
-        url = reverse('admin:%s_%s_change' % (obj._meta.app_label,
-                                              obj._meta.model_name),
-                      args=[obj.id])
-        return format_html(u'<a href="{}">Éditer</a>', url)
-    show_change_link.short_description = 'Éditer ?'
+                       'commitment', 'offer')
+
+    show_change_link = True
 
     def has_add_permission(self, request, obj=None):
         return False