align types between sqlite & postgres

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

migrations/sqlite/2018-04-23-142746_create_post_authors/up.sql

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