If the node is not able to close properly, then the socket disconnection
was not happening nicely. This patch catches errors from a node closing
and ensure the socket disconnection all the time.

When a normal close is attempted and it fails because of several reasons
(it is likely to be because the client might already be disconnected and
then it is not possible to send messages to it), then this situation is
not considered as an error.

Indeed, the client has sent a normal close but it does not listen for
the confirmation from the server. This is a protocol error but happening
on closing.

So instead of getting an exception when closing, catching this exception
later and then firing an `error` event, this makes more sense to catch
the exception, do nothing with it, and fire the expected `close` event.

Firing the close event before closing allows for the user to grab the connection's unique ID. This is useful if the user is keeping track of connection based states across the lifecycle of a connection (e.g. session data).

Fire the event on a protocol error as well before closing.

Define minimal supported version of PHP needed into composer.json

Address hoaproject/Central#70