Installation fails if pg_config is not present

[akshaymankar]

I'm trying to use dbmigrations with sqlite. It should not require postgresql to be installed my system.
Or am I missing something?

[jtdaugherty]

This issue comes up for users of other backends, too, and it's something I'd like to address. You can read some existing discussion on #28, and I'll close this as a duplicate - please pick things up there. Thank you!