Fix hostname logging for Local backend

Both `Formatter::Pretty` and `Formatter::SimpleText` print the hostname along with each command being executed. But the Local backend doesn't have a `@host`, so the output looks like `running [...] on `.

This hard-codes a null `Host` object into `Backend::Local`, which is only used for logging. It's not the most elegant solution, but it's a one-line change.

#### Before

![before.png](http://cl.ly/image/1X170u1q3S2S/Image%202014-05-08%20at%204.59.35%20PM.png)

#### After

![after.png](http://cl.ly/image/3J0F0z313P1k/Image%202014-05-08%20at%205.00.03%20PM.png)