Trying to fix Travis 8 (npm need to be at least 4.0.0 to work)

Author: Lionel Bijaoui

.travis.yml

@@ -8,6 +8,9 @@ node_js:
   - "8"
   - "6"
 
+before_install:
+  - npm i -g npm@">=4.0.0"
+
 install:
   - npm install