Fix/facter 2/home expansion bug

This fixes [FACT-190](https://tickets.puppetlabs.com/browse/FACT-190).

One way to reproduce the issue is by running:

```shell
$ unset HOME
$ facter
/Users/cassiano/.gem/ruby/2.1.1/gems/facter-2.0.1-universal-darwin/lib/facter/util/config.rb:49:in `expand_path': couldn't find HOME environment -- expanding `~' (ArgumentError)
	from /Users/cassiano/.gem/ruby/2.1.1/gems/facter-2.0.1-universal-darwin/lib/facter/util/config.rb:49:in `setup_default_ext_facts_dirs'
	from /Users/cassiano/.gem/ruby/2.1.1/gems/facter-2.0.1-universal-darwin/lib/facter/util/config.rb:60:in `<module:Config>'
	from /Users/cassiano/.gem/ruby/2.1.1/gems/facter-2.0.1-universal-darwin/lib/facter/util/config.rb:5:in `<top (required)>'
	from /Users/cassiano/.rubies/ruby-2.1.1/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:73:in `require'
	from /Users/cassiano/.rubies/ruby-2.1.1/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:73:in `require'
	from /Users/cassiano/.gem/ruby/2.1.1/gems/facter-2.0.1-universal-darwin/lib/facter/util/resolution.rb:2:in `<top (required)>'
	from /Users/cassiano/.rubies/ruby-2.1.1/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:73:in `require'
	from /Users/cassiano/.rubies/ruby-2.1.1/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:73:in `require'
	from /Users/cassiano/.gem/ruby/2.1.1/gems/facter-2.0.1-universal-darwin/lib/facter/util/fact.rb:2:in `<top (required)>'
	from /Users/cassiano/.rubies/ruby-2.1.1/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:73:in `require'
	from /Users/cassiano/.rubies/ruby-2.1.1/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:73:in `require'
	from /Users/cassiano/.gem/ruby/2.1.1/gems/facter-2.0.1-universal-darwin/lib/facter.rb:35:in `<module:Facter>'
	from /Users/cassiano/.gem/ruby/2.1.1/gems/facter-2.0.1-universal-darwin/lib/facter.rb:30:in `<top (required)>'
	from /Users/cassiano/.rubies/ruby-2.1.1/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:73:in `require'
	from /Users/cassiano/.rubies/ruby-2.1.1/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:73:in `require'
	from /Users/cassiano/.gem/ruby/2.1.1/gems/facter-2.0.1-universal-darwin/lib/facter/application.rb:2:in `<top (required)>'
	from /Users/cassiano/.rubies/ruby-2.1.1/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:73:in `require'
	from /Users/cassiano/.rubies/ruby-2.1.1/lib/ruby/2.1.0/rubygems/core_ext/kernel_require.rb:73:in `require'
	from /Users/cassiano/.gem/ruby/2.1.1/gems/facter-2.0.1-universal-darwin/bin/facter:14:in `<top (required)>'
	from /Users/cassiano/.gem/ruby/2.1.1/bin/facter:23:in `load'
	from /Users/cassiano/.gem/ruby/2.1.1/bin/facter:23:in `<main>'
```

If `ENV['HOME']` is unset this will be expanded to the empty string, so this will be expanded relative to the root directory. Facter will not crash but it won't find the right directory either.

For example:

```
└> unset HOME                                        
└> ruby -e "puts File.expand_path(File.join('', '.facter', 'facts.d'))"
/.facter/facts.d
```