internal/dinosql: ignore golang-migrate rollbacks

[treuherz]

We use golang-migrate for migration management, and would like to be able to use it with sqlc. The obstacle here is that it doesn't indicate up/down migrations through special comments, but by naming the SQL files things like

1481574547_create_users_table.up.sql
1481574547_create_users_table.down.sql

so I've had to put the check in ReadSQLFiles and test the functionality differently.

Let me know if you'd like any of this done differently, I'm happy to keep tweaking it.

[kyleconroy]

@treuherz Thanks! Especially for the tests

[treuherz]

Happy to contribute, thanks for the speedy merge!