simplify travis CI configuration

.travis.yml

@@ -5,11 +5,8 @@ php:
   - 5.6
   - 7.0
 
-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
+  - 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 4
 
 before_script:
   - composer install -n