Update to 1.4.24

Upstream changes:
 2 February 2016: statmod 1.4.24

- speedup for rinvgauss() by replacing rchisq() with rnorm() and
  rbinom() with runif().

- speedup for qinvgauss() by using qgamma as starting approximation
  for very small right tail probabilities, and inverse chisq as
  starting approximation for very small left tail probabilities.

- qinvgauss() now computes Newton step using log probabilities
  and a Taylor series expansion for small steps.  This improves
  accuracy in extreme cases.  The stopping criterion for the Newton
  iteration has been revised.

- Bug fix to dinvgauss(), pinvgauss() and qinvgauss() which were not
  preserving attributes of the first argument.

30 December 2015: statmod 1.4.23

- qinvgauss() has been improved to return best achievable machine
  accuracy.  It now checks for backtracking of the Newton iteration.

- dinvgauss() and pinvgauss() now check for a wider range of special
  cases.  This allows them to give valid results in some cases
  for infinite or missing parameter values and for x outside the
  support of the distribution.

26 October 2015: statmod 1.4.22

- Functions needed from the stats and graphics packages are now
  explicitly imported into the package NAMESPACE.


(wen)