Add top_industry_contributors method to TransparencyData Aggregate class

Added GovKit::TransparencyData::Aggregate#top_industry_contributors. 

It's not in the documentation (which I'm told is being overhauled), but the endpoint exists and 
is used by the [python wrapper](https://github.com/sunlightlabs/python-transparencydata/blob/master/influenceexplorer.py#L287). 

I also added response fixture and some tests.