Exclude a problematic spec when run in CRuby via make test-spec until fixed

spec/ruby/library/drb/start_service_spec.rb

@@ -1,8 +1,12 @@
 require_relative '../../spec_helper'
-require_relative 'fixtures/test_server'
-require 'drb'
 
-describe "DRb.start_service" do
+# This does not work yet when run in CRuby via make test-spec:
+# Gem::MissingSpecError: Could not find 'ruby2_keywords' (>= 0) among 28 total gem(s)
+guard_not -> { MSpecScript.instance_variable_defined?(:@testing_ruby) } do
+  require_relative 'fixtures/test_server'
+  require 'drb'
+
+  describe "DRb.start_service" do
     before :each do
       @server = DRb.start_service("druby://localhost:0", TestServer.new)
     end
@@ -25,4 +29,5 @@ describe "DRb.start_service" do
         i+1
       end.should == 4
     end
+  end
 end