if you add the --fail-under switch to the coverage report command, it will exit with a non-zero exit code (which travis will see as a failure) if the code coverage is below the given percentage.
That would make the script section of your .travis.yml file look like:
- coverage run --source="mytestmodule" setup.py test
- coverage report --fail-under=80
Of course you could replace 80 with whatever percentage you'd like.