align types between sqlite & postgres

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

migrations/sqlite/2018-04-22-093322_create_instances/up.sql

@@ -1,6 +1,6 @@
 -- Your SQL goes here
 CREATE TABLE instances (
-    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    id INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT,
     public_domain VARCHAR NOT NULL,
     name VARCHAR NOT NULL,
     local BOOLEAN NOT NULL DEFAULT 'f',