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/2018-05-10-154336_create_likes/up.sql

@@ -4,5 +4,5 @@ CREATE TABLE likes (
     user_id INTEGER REFERENCES users(id) ON DELETE CASCADE NOT NULL,
     post_id INTEGER REFERENCES posts(id) ON DELETE CASCADE NOT NULL,
     ap_url VARCHAR NOT NULL default '',
-    creation_date INTEGER NOT NULL DEFAULT CURRENT_TIMESTAMP
+    creation_date DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP
 )