Updated node version env var for nvm install

Author: Simon Hofmann

.travis.yml

@@ -33,7 +33,7 @@ addons:
 
 before_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
+  - nvm install $node_js
   - PATH=$PATH:`pwd`/node_modules/.bin
 
   - node --version