Fixed random() method of NegativeBinomial (#1255)

* Fixed random method of Negative Binomial: scale = 1/beta, where beta=alpha/mu

Author: Volodymyrk

pymc3/distributions/discrete.py

@@ -240,7 +240,7 @@ def __init__(self, mu, alpha, *args, **kwargs):
 
     def random(self, point=None, size=None, repeat=None):
         mu, alpha = draw_values([self.mu, self.alpha], point=point)
-        g = generate_samples(stats.gamma.rvs, alpha, scale=alpha / mu,
+        g = generate_samples(stats.gamma.rvs, alpha, scale=mu / alpha,
                              dist_shape=self.shape,
                              size=size)
         g[g == 0] = np.finfo(float).eps  # Just in case