Log Message:
Update to 1.4.26

Upstream changes:
28 August 2016: statmod 1.4.26

- Fortran function gaussq2 updated to Fortran 77.

 5 August 2016: statmod 1.4.25

- Add CITATION file.

- pinvgauss() now uses an asymptotic approximation to compute right
  tail probabilities for extreme large quantiles.  This allows it to
  give correct right tail probabilities for virtually any quantile.

- Fix to qinvgauss() to avoid NA values when computing extreme tail
  quantiles where the inverse Gaussian density is subject to floating
  underflow.

- Bug fix to qresiduals() and qresid.invgauss() for the inverse
  Guassian distribution.