align types between sqlite & postgres

this one's achieved by adding "NOT NULL" at the *correct* position in
SQLite's create tables.

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

@@ -1,6 +1,6 @@
 -- Your SQL goes here
 CREATE TABLE notifications (
-    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    id INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT,
     user_id INTEGER REFERENCES users(id) ON DELETE CASCADE NOT NULL,
     creation_date DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
     kind VARCHAR NOT NULL DEFAULT 'unknown',