Installation as root

If you try to install Ghost as root you'll have a very frustrating time of it!

Mostly because grunt-shell < 0.7.0 doesn't pass errors upstream by default - so `grunt init` appears to succeed (even though the bower shell task will have failed), `npm start` works and you'll make it to the landing page but visiting /ghost yields a blank screen.

The actual reason things break is because bower can't run as root.

Opinion time: Ghost itself places no restrictions on running as root (indeed neither do node, npm or grunt-cli) so why should bower be any different?

This patch addresses both issues. If you disagree with my opinion I still recommend updating grunt-shell to 0.7.0 so that at least failure can be properly blamed on bower's insubordination.