[core] Updates Rakefile to use Fog::VERSION

No longer need to read in `lib/fog` to access the VERSION mostly because
we are already requiring it anyway in the Rakefile!

The #version method just repeats Fog::VERSION

`rake -T` works and changelog generates

Rakefile

@@ -16,8 +16,7 @@ def name
 end
 
 def version
-  line = File.read("lib/#{name}.rb")[/^\s*VERSION\s*=\s*.*/]
-  line.match(/.*VERSION\s*=\s*['"](.*)['"]/)[1]
+  Fog::VERSION
 end
 
 def date