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'
Maybe to keep it fairly simple we can check to see if Fog::VERSION is defined and just include it if it is?