Installation as root fails mysteriously

### Issue Summary

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, grunt-cli or pretty much any other program I've heard of) so why does bower get to be an exception?

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 blame for failure can be properly placed on bower's insubordination.

### Steps to Reproduce

1. be root
2. clone the repo
3. `git checkout stable`
4. `npm install`
5. `grunt init`
6. `npm start`
7. visit in browser and experience broken things

### Technical details

* Ghost Version: stable - latest commit:  4e664f0e1c55c11b46a89448f14cc9b059fcad06
* Client OS: Mac OS X 10.9.4
* Server OS: Arch Linux lxc container
* Node Version: 0.10.29
* Browser: Chrome 35.0.1916.153
* Database: SQLite