UPDATED: Use npm v6 instead of v5 in Travis to speed up the package installation.

Author: balassy

.travis.yml

@@ -5,7 +5,7 @@ node_js:
   - "8"
 
 before_install:
-  - npm install -g npm@5
+  - npm install -g npm@6
   - npm install -g serverless
   - npm install -g greenkeeper-lockfile@1