Adds 'connection' to postgres connection user callback

[vinnyjth]

Fixes an error I ran into w/ knex. Knex expects both the err and the connection argument.

https://github.com/knex/knex/blob/master/lib/dialects/postgres/index.js#L111

Stacktrace I ran into when integrating into my app:

TypeError Cannot read property 'on' of undefined 
    node_modules/knex/lib/dialects/postgres/index.js:116:20 
    node_modules/@scout_apm/scout-apm/dist/lib/integrations/pg.js:39:25 Client.originalConnectFn.apply.err [as _connectionCallback]
    node_modules/pg/lib/client.js:270:14 Client._handleReadyForQuery

I'm not familiar with your testing framework. Is this something you want tests for?

[ghost]

Hey @vinnyjth thanks for finding this! How do you feel about using something like userCallback.apply(this, arguments) and changing the arrow function to a normal function?

[EDIT] Also I'd love to get a chance to make sure that the traces are working for you while using knex! Please let me know if you run into any other issues (you could join the slack or just make an issue/PR as well!)