build: install markdown linter for travis

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

PR-URL: https://github.com/nodejs/node/pull/21215
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>

.travis.yml

@@ -7,6 +7,8 @@ matrix:
   include:
     - os: linux
       node_js: "latest"
+      install:
+        - NODE=$(which node) make lint-md-build
       script:
         - NODE=$(which node) make lint-ci
     - os: linux