update npm before travis run script

Author: freedomlang

.travis.yml

@@ -1,6 +1,8 @@
 language: node_js
 node_js:
   - "8.10.0"
+before_install:
+  - npm install -g npm@latest
 script:
   - npm cache verify
   - ./node_modules/.bin/lerna exec -- npm cache verify