The gamrnd fallback under matlab/missing/stats/ does not work under Octave
because the +gamrnd/ folder is not accessible (it has the same name as the
function, which does not work under Octave).

Instead of fixing this, rather make the statistics toolbox a requirement, since
anyways it is very easy to obtain under Octave.

Accordingly:
- do not try to run the unit tests of matlab/missing/stats/ under Octave
- merge the matlab/missing/stats-matlab/ into matlab/missing/stats/, since this
  directory is now MATLAB-only.

Also:
- move matlab/distributions/+gamrnd/ under matlab/missing/stats/ for
  consistency
- in the manual: remove obsolete link to Octave downloads on the Dynare
  website; update URL of Octave Forge

Closes #1638