Allow connecting to redis through unix path in async mode

The async mode doesn't accept a Unix path as a Redis URL. i.e:

unix:///tmp/redis_unix.6379.sock

However, tests pass because they use a Mock that connects to some default TCP urls:

https://github.com/3scale/apisonator/blob/8e7ff4f28088a0b78c173c90ad267524e532b76a/test/test_helpers/storage.rb#L28

DEFAULT_NODES = ["127.0.0.1:7379", "127.0.0.1:7380"].freeze

When done, remember to add a test to storage_async_test.rb. Like:

def test_redis_unix
  storage = StorageAsync::Client.send :new, url('unix:///tmp/redis_unix.6379.sock')
  assert_connection(storage)
end