Load migrations function namespace before calling core/connection multi

I really like ragtimes' approach.  I'm starting to use ragtime with mongodb.

One minor issue I run into when using `lein ragtime` is that I must pass `-r myns` in order to load the namespace that implements the `core/connection` multimethod.

Now, the namespace that holds my migrations function already depends on the namespace that implements `connection` for mongodb. So, simply resolving the migration function before calling `connection` solves my problem.

I also see that ragtime lein  has [`-r ragtime.sql.database`  hardcoded](https://github.com/weavejester/ragtime/blob/master/ragtime.lein/src/leiningen/ragtime.clj#L18) in there. I think that if ragtime.sql.files required `ragtime.sql.database` (which is not too bad I think) then this hardcoding could be removed. If you agree, I'd be happy to also make this change.

Thanks,
Fer