Add __toString method to ConnectionInterface

[Daaarkling]

No description provided.

[nrk]

I'm closing this PR because, aside from missing any kind of explanation for requiring such change, it breaks the whole library since it simply moves the contract of requiring __toString() up in the inheritance tree but doesn't make any change to take care of classes that now have to implement this method to satisfy the new contract.

To explain thins a bit, only single node connections (Predis\Connection\NodeConnectionInterface) require __toString() as this method is used through the library to transparently cast connection parameters into an $ip:port pair.

Moving this requirement up to Predis\Connection\ConnectionInterface means to have this method implemented also for aggregate connections implementing Predis\Connection\AggregateConnectionInterface which doesn't make sense since they don't match a specific $ip:$port pair).