Pull DATABASE_URL from ENV as well as .env and .env.test

Basically if `DATABASE_URL` is available in `ENV`, it's used, otherwise fall back on `.env` and `.env.test`.

The downside is there some's potential for confusion when users are in a development terminal and have `DATABASE_URL` set to something, and don't understand why normal migrations aren't happening (`unset DATABASE_URL` will do the trick).

Fixes #68.