Unhandled Errno::EINVAL

Hello,

I've run into a similar issue to the error handling encapsulatation as issue [353](https://github.com/redis/redis-rb/issues/353). When using hiredis-rb and a connection can't be made during the establish_connection call a Errno::EINVAL is not being captured and raised as a Redis::ConnectionError. Would it make sense to go ahead and make this change? If so I can submit a patch for this. 

```
Errno::EINVAL - Invalid argument:
  redis (3.0.4) lib/redis/connection/hiredis.rb:16:in `connect'
  redis (3.0.4) lib/redis/client.rb:271:in `establish_connection
````