align types between sqlite & postgres

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

migrations/sqlite/2018-06-20-175532_create_mentions/up.sql

@@ -1,6 +1,6 @@
 -- Your SQL goes here
 CREATE TABLE mentions (
-    id INTEGER PRIMARY KEY AUTOINCREMENT,
+    id INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT,
     mentioned_id INTEGER REFERENCES users(id) ON DELETE CASCADE NOT NULL,
     post_id INTEGER REFERENCES posts(id) ON DELETE CASCADE,
     comment_id INTEGER REFERENCES comments(id) ON DELETE CASCADE,