improving npm installation performance

.travis.yml

@@ -9,7 +9,8 @@ php:
 
 # as this is a php project, node.js v4 (for JS unit testing) isn't installed
 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 4
+  - '[ ! -d "$HOME/.nvm" ] && mkdir -p $HOME/.nvm && curl -o- https://raw.githubusercontent.com/creationix/nvm/v0.33.8/install.sh | NVM_METHOD=script bash'
+  - source ~/.nvm/nvm.sh && nvm install 4
 
 before_script:
   - composer install -n
@@ -31,5 +32,6 @@ cache:
   directories:
     - $HOME/.composer/cache/files
     - $HOME/.composer/cache/vcs
+    - $HOME/.nvm
     - $HOME/.npm
     - js/node_modules