Replace Fog version constant

A mistake in `fog-core` removed `Fog::VERSION` in a minor update which
meant the HTTP headers broke.

This uses the replacement `Fog::Core::VERSION` for the moment.

There was an issue with that originally which was why the original was
left in place but hopefully this update will be the end of the matter.
Merge pull request #65 from brightbox/replace_version_constant

Replace Fog version constant