Use the latest version of node

stylelint requires node >=10, so in order to test that generator we need
to install a more recent version.

.travis.yml

@@ -2,7 +2,7 @@ language: ruby
 rvm: 2.6.5
 cache: bundler
 before_install:
-  - nvm install 8.16.0
+  - nvm install 13.12.0
   - git config --global user.name 'Travis CI'
   - git config --global user.email '[email protected]'
   - gem update --system