do not define rspec task if rspec is unavailable

lib/tasks/tests.rake

@@ -11,9 +11,11 @@ namespace :ci do
   end
 end
 
+if defined?(RSpec)
   namespace :tests do
     desc "Run all specs that generate fixtures for rspec or jasmine"
     RSpec::Core::RakeTask.new(:generate_fixtures) do |t|
       t.rspec_opts = ['--tag fixture']
     end
   end
+end