Add "predis.client_constructor".

It should be used only to override the default behaviour of creating
an instance of Predis\Client when constructing a new client object,
so that developers can return their own subclass or implementation of
Predis\ClientInterface.
Use the URI parser defined in Predis\Connection\ConnectionParameters.

This change requires a bump of the minimum required version of Predis
to v0.8.3.
Bump VERSION.