Add Predis support (#59)

* wip

* Add Predis support

* Enable phpredis extension only for some builds

* Add note about phpredis

* Fix if command

* Revert travis changes & add a version check to predis tests

src/Exceptions/PhpRedisNotInstalledException.php

@@ -0,0 +1,8 @@
+<?php
+
+namespace Stancl\Tenancy\Exceptions;
+
+class PhpRedisNotInstalledException extends \Exception
+{
+    protected $message = 'PhpRedis is not installed. PhpRedis is required for Redis multi-tenancy because Predis does not support prefixes.';
+}