align types between sqlite & postgres

use DATETIME for SQLite's time type. This way, Diesel picks up on what
it's supposed to be.

migrations/sqlite/2018-05-13-122311_create_notifications/up.sql

@@ -2,7 +2,7 @@
 CREATE TABLE notifications (
     id INTEGER PRIMARY KEY AUTOINCREMENT,
     user_id INTEGER REFERENCES users(id) ON DELETE CASCADE NOT NULL,
-    creation_date INTEGER NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    creation_date DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
     kind VARCHAR NOT NULL DEFAULT 'unknown',
     object_id INTEGER NOT NULL DEFAULT 0
 )