Don't try to load config from home directory if HOME isn't set

It's possible the HOME environment variable may not be set. jbundler shouldn't crash if it isn't.
Ugh gross, I forgot about this stupid Ruby behavior, e.g.:

```
1.9.3-p484 :001 > x
NameError: undefined local variable or method `x' for main:Object
	from (irb):1
	from /home/tyler/.rvm/rubies/ruby-1.9.3-p484/bin/irb:12:in `<main>'
1.9.3-p484 :002 > if false
1.9.3-p484 :003?>   x = 1
1.9.3-p484 :004?>   end
 => nil 
1.9.3-p484 :005 > x
 => nil 
1.9.3-p484 :006 > 
```

I'd go with the first route and be more explicit about it.