Make operating without postgresql easier

[arodland]

If a user doesn't want to use the remember or lastsaw modules, they should be able to operate without a postgres DB or any of the related libraries.

Right now it's possible, but you have to do this:

• Remove requires 'Mojo::Pg'; from cpanfile.
• Remove use Mojo::Pg; from Borkbot.pm.
• Remove any modules that call $bot->pg from the config.

It should be possible without any source editing.