Remove unf warning

The warning that the 'unf' gem could not be loaded can be easily fixed by explicitly making 'unf' a dependency for mulder.

See http://stackoverflow.com/questions/19666226/warning-with-fog-and-aws-unable-to-load-the-unf-gem for info.
Merge pull request #8 from shopkeep/remote-unf-warning

Remove unf warning