switch to newer node version, in an attempt to support mocha 6

.travis.yml

@@ -11,7 +11,7 @@ php:
 # as this is a php project, node.js v4 (for JS unit testing) isn't installed
 install:
   - if [ ! -d "$HOME/.nvm" ]; then mkdir -p $HOME/.nvm && curl -o- https://raw.githubusercontent.com/creationix/nvm/v0.33.8/install.sh | NVM_METHOD=script bash; fi
-  - source ~/.nvm/nvm.sh && nvm install 4
+  - source ~/.nvm/nvm.sh && nvm install 10
 
 before_script:
   - composer install -n