Fix cacheClass in full example in README.md

[richard67]

When using

use Heise\Shariff\LaminasCache;
/**
 * Sample configuration
 *
 * @var array
 */
private static $configuration = [
    'cacheClass' => LaminasCache::class,
...

like currently written in the full example in README.md, I get a PHP error about class not being found.

When using

use Heise\Shariff\LaminasCache;
/**
 * Sample configuration
 *
 * @var array
 */
private static $configuration = [
    'cacheClass' => 'Heise\\Shariff\\LaminasCache',
...

it works.

This PR corrects the full example in README.md to reflect this.

When not specifying the cache class in index.php, everything works, too, so nothing else to be changed, Backend.php is correct.

[richard67]

Travis error seems not to be related to this PR.

[richard67]

Ping @compeak @sehkunde .

[compeak]

Thanks.

[richard67]

Thanks.