correcting syntax of npm install

.travis.yml

@@ -9,7 +9,7 @@ php:
 
 # as this is a php project, node.js v4 (for JS unit testing) isn't installed
 install:
-  - '[ ! -d "$HOME/.nvm" ] && mkdir -p $HOME/.nvm && curl -o- https://raw.githubusercontent.com/creationix/nvm/v0.33.8/install.sh | NVM_METHOD=script bash'
+  - if [ ! -d "$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
 
 before_script: