Added option redundant_servers

[Nyholm]

This will fix php-cache/issues#69

Ping @panki3a, Can you test this patch?

cache_adapter:
  providers:
    memcached:
      factory: 'cache.factory.memcached'
      options:
        persistent_id: 'story_pool'
        host: 'foo'
        port: 123
        redundant_servers:
          - host: 'bar'
            post: 123
          - host: 'baz'
            post: 123

[codecov-io]

Current coverage is 21.90% (diff: 0.00%)

Merging #48 into master will decrease coverage by 0.86%

@@             master        #48   diff @@
==========================================
  Files            35         35          
  Lines           606        630    +24   
  Methods          77         77          
  Messages          0          0          
  Branches          0          0          
==========================================
  Hits            138        138          
- Misses          468        492    +24   
  Partials          0          0          

Powered by Codecov. Last update 775dabc...0362f8b

[panki3a]

i tested it, and its functional.

[Nyholm]

Thank you for trying this out.