ensure last Firefox Addon-SDK release on Firefox XPI production build

This small change (an updated *grunt-mozilla-addon-sdk*, and small changes to the *Gruntfile.js* and *package.json*) ensures that we are using the last released Mozilla Addon SDK on every ```npm run grunt:xpi:production``` run.

NOTE:
- even without this change, no addon-sdk sources are actually included in the xpi package, nevertheless we're using *cfx* from a defined addon-sdk release and, during the xpi building, it saves the release version inside the package metadata;
  so long story short this change ensures that we always use a *cfx* tool from the very last released package when we build a production xpi using ```npm run grunt:xpi:production```.
- I'm not sure if it is due to a different shell or npm version but I have had to rework a bit the npm scripts to get them to work as expected