fixed compatibility issues with Ruby 1.8.7

This fixes the issue with Ruby 1.8.7 where hash syntax was causing an error:
```ruby 
/var/lib/gems/1.8/gems/redis-sentinel-1.4.3/lib/redis-sentinel/client.rb:143: odd number list for Hash (SyntaxError)
            host: uri.host,
                 ^
```

The old syntax was already in use ([e.g. in here](lib/redis-sentinel/client.rb#L76)) so I've changed the other instances to make it 1.8.7 compatible.