Update npm and add cache

Author: felixfbecker

.travis.yml

@@ -3,6 +3,16 @@ language: node_js
 node_js:
   - '7.9.0'
 
+cache:
+  directories:
+    - $HOME/.npm
+
+before_install:
+  - npm install -g [email protected]
+
+install:
+  - npm ci
+
 script:
   - npm run lint
   - npm run build