fix: default pact specification version to 2

Author: bethesque

lib/pact/consumer/configuration/mock_service.rb

@@ -20,7 +20,7 @@ def initialize name, consumer_name, provider_name
           @port = nil
           @standalone = false
           @verify = true
-          @pact_specification_version = nil
+          @pact_specification_version = '2'
           @finalized = false
         end