fix: Coerce some denominators to rational when calculating inverse beta.

This change fixes a segmentation error produced by BigDecimal gem on
ruby 3.1.2, where it fails to calculate a big float. It was "fixed"
in BigDecimal 3.1.2, but this version might not be compatible with older
ruby versions.

Reported here: https://bugs.ruby-lang.org/issues/18604 & here: ruby/bigdecimal#220
Potential fix here: ruby/bigdecimal@e236c2e

lib/math.rb

@@ -104,9 +104,9 @@ def self.incomplete_beta_function(x, alp, bet)
 
       d = 1.0 + numerator * d
       d = tiny if d.abs < tiny
-      d = 1.0 / d
+      d = 1.0 / d.to_r
 
-      c = 1.0 + numerator / c
+      c = 1.0 + numerator / c.to_r
       c = tiny if c.abs < tiny
 
       cd = (c*d).freeze