Update .travis.yml

Author: tbolis

.travis.yml

@@ -1,7 +1,7 @@
 before_install:
   - sudo apt-get -qq update
   - sudo apt-get install -y libcairo2-dev libjpeg-dev libgif-dev npm nodejs libjpeg8-dev libpango1.0-dev libgif-dev build-essential g++
-  - sudo npm -g update npm
+  - if [[ `npm -v` != 3* ]]; then npm i -g npm@3; fi
 
 language: node_js
 node_js: