build: install markdown linter for travis

Looking at the Travis logs, e.g. https://travis-ci.com/nodejs/node/jobs/126938119 from https://github.com/nodejs/node/pull/21059:

...
$ NODE=$(which node) make lint-ci
Running JS linter...
Running C++ linter...
Total errors found: 0
The markdown linter is not installed.
To install (requires internet access) run: make lint-md-build
...

Run make lint-md-build to install the markdown linter.

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