Use a modern nodejs version on travis. On the default version the build process broke due to missing node version.

.travis.yml

@@ -14,6 +14,10 @@ php:
   - '7.3'
   - '7.4'
 
+before_install:
+  # Use a more modern nodejs version, than provided by default
+  - nvm install --lts
+
 install:
   - composer install -o
   # Compile assets