Use package with smaller size in vignettes

[andrie]

The vignettes uses MASS as an example.

The download size of MASS is 476K bytes (https://cran.r-project.org/src/contrib/).

The package darts is only 6.8K in size, and still needs compilation, but also doesn't have any other dependencies.

This switch will make a difference in download and compilation time during vignette building.