I found my script: section in .travis.yml being ignored and blame it on providing an empty install: section.
https://travis-ci.org/andreasabel/agda-scope/jobs/575524258/config
The paradoxical error message was
No DESCRIPTION file found, user must supply their own install and script steps
making me feel being fooled with.
I discovered a workaround by accident: putting a bogus instruction into the install: section, like an echo statement.