chore(travis): blow away cached node_modules when npm-shrinkwrap.json changes

`npm install` blindly accepts the node_modules cache and doesn't verify if it matches requirements in the current npm-shrinkwrap.json.

This means that if we are using travis cache and npm-shrinkwrap.json changes npm will keep on using the old dependencies, in spite of the guarantees that shrinkwrap claims to offer.

#11110 (comment)

With this change, we will blow away the node_modules directory if the shrinkwrap changes compared to the one
used to populate node_modules.

Author: IgorMinar

.travis.yml

@@ -45,7 +45,9 @@ install:
   # Log HTTP requests
   - npm config set loglevel http
   - npm install -g npm@2.5
+  - diff -q npm-shrinkwrap.json node_modules/npm-shrinkwrap.cached.json; if [[ $? -ne 0 ]]; then echo "Blowing away node_modules"; rm -rf node_modules; fi
   - time npm install
+  - cp npm-shrinkwrap.json node_modules/npm-shrinkwrap.cached.json
 
 before_script:
   - mkdir -p $LOGS_DIR