Log Message:
Update to 1.4.21

Upstream changes:
30 March 2015: statmod 1.4.21

- qinvgauss() now treats input arguments of different lengths or NA
  parameter values more carefully.

- elda() now gracefully removes structural zeros, i.e., rows where
  the number of cells or the number of assays is zero.

- S3 print and plot methods for "limdil" class now registered.

- Use of require("tweedie") in the qres.tweedie() code replaced by
  requireNameSpace("tweedie").