With heroku-buildpack-apt, this buildpack enables running
on Heroku. To use it, ensure that
heroku-buildpack-apt comes earlier than this in the buildpacks list, and include
Some inspiration (for dealing with Xvfb's compiled-in
XKB_BIN_DIRECTORY, which leads to it complaining that it can't find
/usr/bin/xkbcomp) comes from fxtentacle's answer to http://stackoverflow.com/a/34219781/1162903.
Copy the snippet above into CLI.