Build/Test Tools: On Travis CI install and use the node version which is specified in package.json.

Props netweb.
See #35105.

git-svn-id: https://develop.svn.wordpress.org/trunk@40258 602fd350-edb4-49c9-b593-d223f7449a82

.travis.yml

@@ -59,7 +59,9 @@ before_script:
   elif [[ ${TRAVIS_PHP_VERSION:0:3} != "5.2" ]]; then
     composer global require "phpunit/phpunit=4.8.*"
   fi
-- npm install -g npm
+- npm --version
+- node --version
+- nvm install 6.9.1
 - npm install -g grunt-cli
 - npm install
 - npm prune