try to install node 5.6 on travis

.travis.yml

@@ -33,7 +33,7 @@ install:
     - sudo apt-get update
     - sudo apt-get install -o DPkg::Options::="--force-confold" -y libxml2 libxml2-dev libxml2-utils python3-jinja2 python3-setuptools apache2 libapache2-mod-fastcgi firefox curl
     - sudo easy_install3 requests ocdev
-    - nvm install 5.0
+    - nvm install 5.6
     - npm install -g npm@latest
     # install php modules
     - wget https://scrutinizer-ci.com/ocular.phar