Explicitly requiring faye in 'lib/restforce/concerns/streaming.rb'

Hello there, and thank you for the neat library!  It's helped me so far with my evaluation of the Salesforce Streaming API a great deal, and I can't wait to integrate it further :)

While evaluating `restforce`, I noticed that my sample ruby application would error out upon connection with the following error:

```
uninitialized constant Restforce::Concerns::Streaming::Faye (NameError)
```

The `README` documentation seems to indicate that requiring `faye` is standard procedure for streaming operations, but it seems odd to me to have the user require a dependency in their code to satisfy an internal implementation detail.

This changeset resolves this issue by explicitly requiring the `faye` dependency in `lib/restforce/concerns/streaming.rb`.  

If merged, users of `restforce` should now only have to require `restforce` to preform streaming operations.