Fix markdownlint test

[waylan]

Just experimenting trying to understand why the markdownlint test is failing.

[waylan]

So Travis is using Node.js version v8.9.1, but markdownlint-cli dropped support for version 8 in 0.22.0 (see igorshubovych/markdownlint-cli#93). Apparently, Node doesn't prevent itself from installing incompatible versions as markdownlint-cli version 0.23.0 is installed.

We could force an older version with npm install -g markdownlint-cli@0.21.0 but I'd rather using a newer version of Node. The Travis docs only list version 12 through 14, so I don't know why we are getting 8.

[waylan]

Sigh. So if I set the language to node.js I can get Travis to use a more recent version, but then we run into weird Python errors and we need Python to run tox. I guess I could run the test directly from Travis rather than using tox, but that would be inconsistent with the rest of our tests.