set expon_rate just once, as exponential not gamma

I couldn't compile and I noticed there were a couple funny lines in PolyaGammaStd.cpp. In particular

1. `this->expon_rate` was being set twice, once as a gamma and once as an exponential, and
2. the constructor calls for `exponential_distribution` and `gamma_distribution` may have been switched, since they seem to have the wrong numbers of arguments

I just made it an `exponential_distribution`. Pull if that makes sense!