It looks like Travis-ci doesn't actually support multiple languages in one repository. So, even though we can set a differing versions of python for a node_js project and visa versa, it actually only picks one version of the other language.
However, it does support running apt-get as part of a job and running a matrix of env vars. So it is likely we could run tests on at least a version python2 and python3, or pick different versions of node to install as part of the travis-ci scripts.
For now I'll set it to run on, several versions of python as that seems to be the less covered path.