rgdal travis error: configure: error: gdal-config not found or not executable.

Hi,

I'm testing a package using travis and I keep getting an error message
about rgdal not being found or executable. I've been trying to piece
together things I've found online but to no avail.

https://travis-ci.org/github/josephlewis/leastcostpath/builds/663944787

travis.yml

# R for travis: see documentation at
https://docs.travis-ci.com/user/languages/r

language: R
sudo: false
cache: packages

repos:
  CRAN: https://cran.rstudio.com
  rforge: http://R-Forge.R-project.org

apt_packages:
    - gdal-bin

before_install:
install.packages("rgdal", repos="http://R-Forge.R-project.org")


Any help would be appreciated.

Thanks.

Kind regards,
Joseph Lewis