Kerberos hash comments has :provider, but :principal is passed

This may be a case where the comments are out-of-date.

In [impala.rb:39](https://github.com/colinmarc/impala-ruby/blob/master/lib/impala.rb#L39) it has :provider as an entry for the :kerberos hash:

```ruby
# - *:provider* (String)
```

However, later on, in [sasl_transport.rb:25](https://github.com/colinmarc/impala-ruby/blob/master/lib/impala/sasl_transport.rb#L25) ,:principal is actually passed instead of :provider:

```ruby
@gsscli = GSSAPI::Simple.new(@options[:host], @options[:principal])
```