feat: use 0 as the nil pact specification version

Author: bethesque

lib/pact/specification_version.rb

@@ -14,5 +14,5 @@ def after? other
     end
   end
 
-  SpecificationVersion::NIL_VERSION = Pact::SpecificationVersion.new('')
+  SpecificationVersion::NIL_VERSION = Pact::SpecificationVersion.new('0')
 end