update npm version in travis build

RobinBuschmann · RobinBuschmann · commit dc186cd8e635 · 2017-07-04T20:00:19.000+02:00
diff --git a/.travis.yml b/.travis.yml
@@ -3,6 +3,9 @@ language: node_js
 node_js:
   - "7"
 
+before_install:
+  - npm i -g npm@5
+
 install:
   - npm install
 
