(#190) Fix file expansion crash when $HOME is not set.

Without this patch, when facter is run in an environment where $HOME
is not set, it crashes while trying to expand_path "~". The patch
uses Ruby's ENV['HOME'] instead of "~", which sanitises the
expansion.
(#190) Don't set external facts when ENV['HOME'] is nil
Merge branch 'pull-691' into facter-2