Proposal to fix sails www --prod

As stated in here: https://github.com/balderdashy/sails/issues/1973, the command ```sails www``` wasn't working properly in production mode. This PR is a proposal to fix it.

The only thing I did is to, after Sails got all its configuration sorted, execute ```grunt buildProd``` instead of ```grunt build``` when the user override NODE_ENV or execute sails with ```--prod``` option (or use any kind of override for what matters)

```grunt buildProd``` was already doing the job of minifying and concatenate so I guessed it was the target intended for it at first.

And then the control is on user hands as this grunt task can be easily tweaked.

Hope this correspond to the guidelines