Switch to HOME environment variable instead of ~ for populating default home_path

When ~ cannot be resolved it throws an exception which stops execution, even if you've completely overridden the default value:

/opt/rbenv/versions/2.0.0-p353/lib/ruby/gems/2.0.0/gems/berkshelf-api-1.1.1/lib/berkshelf/api/config.rb:16:in `expand_path': couldn't find HOME environment -- expanding `~' (ArgumentError)

When ENV['HOME'] is undefined it fails silently.  I'm not sure that this is necessarily ideal, but it allows the app to run under runit (as used in the berkshelf cookbook).