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>'
```

What would be the "right directory" relative to the user's home if the user's home is not set?

My point is that there's no right path in this case since there's no `HOME` set. Not finding any external facts seems appropriate. If it crashes, on the other hand, then it's useless.

The issue is not about `facter` not finding external facts: it's about `facter` crashing due to bad Ruby practice. This patch fixes the issue as it's reported on [FACT-190](https://tickets.puppetlabs.com/browse/FACT-190).

Also, it fixes the crash without changing behaviour by using Ruby's `ENV` instead leaving the expansion to the shell, which in this case is non-existent.