travis-ci build failed due to missing ghostscript

[vsimko]

...
checking dia ... ok
checking odg ... ok
checking ods ... ok
checking odt ... ok
checking R ... ok
checking svg ... ok
checking pdf ... ok
checking png ... ERROR: Ghostscript not installed (gs command)
Selected output format is: pdf
R script loaded
Creating PDF of size 7 x 7 
Now running R code that should plot graphics to the PDF
R code done.
This PDF contains only a single page - splitting not needed
Cropping PDF: ./example1.pdf ... mv: cannot stat ‘.pdfcrop’: No such file or directory
done
...