gracefully handle missing ENV['HOME'] var

```ENV['HOME']``` isin't always set and if ```File.join``` is passed ```nil``` it throws an error.

This handles missing ```ENV['HOME']``` gracefully and checks the the current directory for the config file.

```ruby
irb(main):001:0> File.join(ENV['HOMESSSS'], '.collins.yml')
TypeError: can't convert nil into String
	from (irb):1:in `join'
	from (irb):1
	from /usr/bin/tumblr_irb:12:in `<main>'
irb(main):002:0> File.join(nil, '.collins.yml')
TypeError: can't convert nil into String
	from (irb):2:in `join'
	from (irb):2
	from /usr/bin/tumblr_irb:12:in `<main>'
```
This duplicates the next entry of explicitly loading ```./.collins.yml```. I think if HOME is missing, we should just return nil, which will get dropped from the array with ```#compact```