Fix gamma parametrization

The gamma random number generator in stan claims to be parametrized by shape and rate.  However, something like

```
parameters {
   real<lower=0, upper=1> fake;
}

model {
}

generated quantities {
   real fred;
   fred <- gamma_rng(1, 3);
}
```

gives fred with a mean of 3 rather than the 1/3 it should be.  [In 

rstan (Version 1.3.0, packaged: 2013-04-12 21:12:02 UTC, GitRev: f57455593d14)

]

This is because the boost gamma is parametrized by shape and scale rather than shape and rate.  This should be the appropriate fix --- I haven't tested this code, so caveat emptor...