em-pg-client 0.3 update

Hello,

New em-pg-client is in town, so it'd be nice to update em-pg-sequel too.

Ad 3811f03 - can't run test with minitest 5.x (see commit message)

Ad 4ea7d02 - prevents pending requests to be left in queue

This might happen if:
1. There was more query requests than pool max_size
2. The connection to server was lost

So, I've ported the solution from PG::EM::ConnectionPool.

Ad 3555511 - I don't think em-synchrony should be a requirement, em-pg-client can do without it and there are [folks](https://github.com/royaltm/ruby-em-pg-client/issues/8) who'd like to avoid using it.

And the last but not least - would you release a new gem please?