Improve build for postgresql

[asyd]

Hi folks,

I would like to use Singularity with a PostgreSQL database. I notice the 0.21 release notes say it is now supported, however the shaded Jar doesn't include jdbc driver.

I'll submit a PR to use a postgresql maven profile to add the jdbc driver as well as the correct migrations.sql.

If you want I can improve the patch to create a mysql profile in the same way.

Thanks for you job folks!