Fixed TrendLeaderboard serialization

Hi,

I played some more with Ducksboard and this library and found out the TrendLeaderboard API wasn't working.
It was due to serialization attributes missing in the model class.
The members would be serialized with their default names (PascalCase) and since Ducksboard requires lowercase JSON attributes, the values wouldn't be picked up.
I have tested these changes in my app for a few days now and it's been working fine.

Cheers
Xavier