Fix conflict with the main Fog gem

The fog gem depends on `fog-provider*` gems that depend on `fog-core`.
Both define the same constant `Fog::Version` which might be conflicting.

@geemus I've got stucked when working with the `fog-xml` gem with those warnings: 

> uninitialized constant Fog::VERSION
        @ /Users/paulo/.rvm/gems/ruby-2.1.2@fog-xml/gems/fog-core-1.24.0/lib/fog/core/connection.rb:40:in `initialize'
This change will make the requisition fail if the gem `fog` is not installed. =s
Maybe we can add the `Fog` version from the gem `fog`?