Fix refresh token. expires_in has date format, not integer

[yaroslawww]

now()+300 returns integer value. And this value can't be compared with string datetime expires_in saved in database (see migration file). So app never uses a refresh token.

Also, reduced supported versions to allow automatically do casts in model.

[rickkuilman]

Ran into the same issue today! I've fixed it by casting expires_in to timestamp, but this solution is cleaner by honoring the database type of DATETIME.