MD5 $cacheId in GSBucket cache handler

I think the $cacheId should be md5'ed otherwise it'll produce strange errors.
See file.php handler, it md5's it so the same should be done here since it works similar way.

See:

```php
function _getCacheId($id, $group)
```

When progressive cache is on the cache handler might be asked to produce $cacheid's for things like:

```
$id = 'http://localhost:8080/index.php'
```

then handler processes something like `file_put_contents('gs://gs-joomla/cache/page/-http://localhost:8080/index.php-en-GB', ....)` which obviously fails with an warning error

Other example:
```
$id = 'http://localhost:8080/index.php_lock'
```
produces `file_put_contents('gs://gs-joomla/cache/page/-http://localhost:8080/index.php_lock-en-GB', ....)`

Another, even worse:
```
$id = 'menu_items{"menutype":"bottommenu","base":"","startLevel":}1,1.101_lock'
```
produces the following wrapper path:
```
file_put_contents('gs://gs-joomla/cache/mod_menu/-menu_items{"menutype":"bottommenu","base":"","startLevel":"1"}1,1.101_lock-en-GB', ....)
```