[tests] Use Predis\Autoload from include_path when not in repository.

This is useful only for running the test suite after installing the
PEAR package, see #126 for reference.

tests/bootstrap.php

@@ -9,7 +9,13 @@
  * file that was distributed with this source code.
  */
 
-require __DIR__.'/../autoload.php';
+if (file_exists(__DIR__.'/../autoload.php')) {
+    require __DIR__.'/../autoload.php';
+} else if (@include('Predis/Autoloader.php')) {
+    Predis\Autoloader::register();
+} else {
+    die('ERROR: Unable to find a suitable mean to register Predis\Autoloader.');
+}
 
 require __DIR__.'/PHPUnit/ArrayHasSameValuesConstraint.php';
 require __DIR__.'/PHPUnit/RedisCommandConstraint.php';