[tests] Use annotations to specify expected Redis server version.

This change targets only method annotations and aims to make the body
of tests more readable using the @requiresRedisVersion annotation.

Tests using this annotation requires to be assigned the "connected"
group of tests because they create a connection to the Redis instance
specified in phpunit.xml to fetch its the version.

This is a quick example of how this annotation can be used:

    /**
     * @group connected
     * @requiresRedisVersion >= 2.8.9
     */
    public function testExecutedOnlyWithMatchingRedisVersion()
    {
    }

Future improvements (currently not needed) include:

  * Same annotation working on a class-level (but still applied only
    to test methods with an explicitly assigned @group connected).

  * Ability to specify a version range.