Redis Client: Use builtin class loader.

#### What's this PR do?
- Gets rid of custom custom anonymous `spl_autoload_register()` callback in `settings.php`
- Defines builtin `Redis_Client_Predis::setPredisAutoload()` as a Predis classloader

#### Where should the reviewer start?
Read [`README.Predis.txt`](/omega8cc/redis/blob/7.x-2.x/README.Predis.txt) of Redis Drupal module.

#### How should this be manually tested?
- Clean all caches: `drush cc all`
- Make sure the frontpage isn't blank
- Log in as admin
- Visit Drupal [status report](http://dev.dosomething.org:8888/admin/reports/status) page
- Make sure Redis monitoring endpoint reports `Connected, using the Predis client.`

#### Any background context you want to provide?
I made this change to Redis loader during experiments with SimpleTest for UK SSO. Originally I tried to fix [incompatibility](https://www.drupal.org/node/1760256) of SimpleTest and Redis module. Unfortunately, this didn't solve the issue (as well as Redis module upgrade).

Nevertheless, I found the change useful as is. I think, `settings.php` shouldn't contain anything more complex than PHP control structures. Especially that's not the place for something as complex as lambda abstractions. 

#### What are the relevant tickets?
Improves #3080