Allows failures to gather remote dependencies to failure silently

A use case exists where Geminabox is used to cache gems locally so as to provide a gem repository when RubyGems is down. To do this, failures to access rubygems to gather dependencies need to fail silently so that only local dependencies are returned.

To use the setting the following should be added to the config.ru:

    Geminabox.allow_remote_failure = true