Update Gemfile reference to prerelease fog-core

Dynamic configuration is based on changes in `fog-core` not currently
released. This references the changes from `master`

Unfortunately thanks to `mime-types` needing it's own Gemfile we have to
shuffle two temp references.
Encapsulate Config in class

`Fog.credentials` is basically a global Hash that holds everything.
Thanks to primitive obsession it means that the configuration is dumb
forcing decentralised code to each know little bits about it.

This hash is dereferenced when passed into a service so any values that
are shared or dynamic are duplicated or can fall out of sync. Examples
being credentials shared by several services of one provider or tokens
that can be revoked and refreshed globally.

`Fog::Brightbox::Config` is a centralised configuration that smartens up
the details and can be initialised itself by legacy credentials.
Use Brightbox::Config within Compute
Raise if required config is missing

For backwards compatibility if required config settings are missing then
an ArgumentError is raised.

This is implemented as a delegate because requirements may vary between
services and we do not want the logic in the Config.
Merge pull request #1 from brightbox/config

Use Brightbox::Config object to configure services