Parse uri string in sentinels array

This is a workaround to allow use with Rails.cache to resolve issue #30. Allows using a uri for sentinels instead of a hash like so:

    Rails.config.cache_store = :redis_store, { master_name: 'master', sentinels: ['sentinel://localhost:26379'] }

Note: reopening the last pull request #31. I had closed it because of a problem, but it turned out to just be my sentinel config.