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.

https://github.com/angular/angular.js/pull/11110#issuecomment-75302946

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