Adding explicit NPM install before deploy

Author: cbetta

.travis.yml

@@ -11,7 +11,9 @@ branches:
   only:
     - master
     - /^v[0-9]/
-before_deploy: npm run docs
+before_deploy:
+  - npm install 
+  - npm run docs
 deploy:
   - provider: pages
     local-dir: docs