trying harder to convince travis CI to use node.js 4

.travis.yml

@@ -4,8 +4,12 @@ php:
   - 5.5
   - 5.6
   - 7.0
-node_js:
-  - "node"
+
+env:
+  - TRAVIS_NODE_VERSION="4"
+
+install:
+  - rm -rf ~/.nvm && git clone https://github.com/creationix/nvm.git ~/.nvm && (cd ~/.nvm && git checkout `git describe --abbrev=0 --tags`) && source ~/.nvm/nvm.sh && nvm install $TRAVIS_NODE_VERSION
 
 before_script:
   - composer install -n